Most people will inevitably focus on motive simply because they want (or need) to see direct results in terms of sales, especially on this site. I don't really want this thread to be about which franchise does better or which franchise sells systems better but just generally about the two concepts and which is the most important. First of all, what do I mean by motive and justification? It might already be clear but just in case: Motive would be the reason people eventually buy a console whilst justification would be the reason people are happy about making that choice.
Motive:
Let's start with the obvious to get it out of the way: console allegiance. These people are unlikely to be effected by justification so there's really no point in discussing it. They would sell there Granny's hip replacement or roll down a hill naked in a barrel full of cacti if it meant being able to hug their beloved console. No justification is needed, just the heady aroma of warm plastic and a familiar start up screen to worship every day.
Now that's dealt with, let's consider motive. I'm a PS4 fan so I may well get the motives wrong for the other two console so feel free to correct me if that's the case. I think two from each will suffice to get my point across. For the PS4 I would say the motives could be Uncharted and Gran Turismo, for the XB1 Halo and Gears of War, and for the WiiU Zelda and Mario. I'm aware that in two of those examples the franchise I've named are not yet available but remember I'm talking about the concept here, so there's little point in reminding me of that.
Of course motive is important. It's probably the single most important factor in the early years of a console's release and each person has their own personal favourite franchise they're waiting for to finally jump on board. Price is also a factor but I'd also point out that price is linked to franchises because I doubt anyone would buy a console that contained nothing that interested them even if that console was only £100 (or $100).
Clearly it's important then and drives the sales of consoles directly.
Justification:
There are many ways a console manufacturer can make sure you feel justified in opting to support them. From constroller to online infrastructure to services offered, but I want to get straight the the meat of the main reason people buy consoles, and that of course is the games. It could be argued otherwise -- I'm aware of that -- but I do think I'm safe to assume 'most' people will agree it's the games.
Picking examples for this category is much harder than in 'motives' because the media may not necessarily cover these games on a daily basis, and unless you follow the community of all consoles closely you're not likely to know what games are pushing the right buttons. I'll offer up examples nonetheless because I want to be clear about what I mean. Perhaps, Rocket League on the PS4, Ori And The Blind Forest on the XB1 and Mario Maker on the WiiU. I'm really not sure about my choice for the WiiU so I'd be interested to see what games are 'filling those gaps' for that fanbase.
I think it becomes clear that smaller games which are more often than not Indie in identity are at the forefront of this catagory. I could easily enlarge the PS4 list of justifications because I'm fully involved in that ecosystem but because I'm not fully aware of the others I've simply offered examples for the overall concept, so please let's not get into the pointless cul-de-sac that is 'what is an Indie game?'. Just think of this in simpler terms: smaller titles that don't make huge headlines on a daily basis but keep the fanbase happy in between the releases of bigger titles. I'm sure once you consider it that way you'd have to agree it's more likely to be Indie games.
Which Is Most Improtant?
It would be easy to assume both are equally important but it's impossible for anything like this to be a 50/50 situation, so even if it's only a 49/51 split the question merits a thoughtful answer. And please, because I've used the very close split as an example, let's not get into a pedantic debate about natural fluctuation. Over time the fluctuation is going to favour one over the other anyway. Because this is my conclusion to the concept of motive versus justifaction and I'm certain many people may well have already noticed I've overlooked something in the above examples for justification, I'm going to mention DLC. Of course long term support must play a a part in justification because it stretches those motives beyond their initial impact, but is it as consistant as smaller titles? Driveclub would be a good example for the PS4, Splatoon would be a good example for the WiiU, but in all honesty I can't think of an example for the XB1. That's not to say there isn't one, it's just to aknowledge my ignorance. However, it doesn't in any way change the question. There will be an answer regardless of what importance you place on smaller titles or DLC and it still falls under the catagory of justification.
Time spent on the console:
I don't think it's contraversial to state a manufacturer of a console will want you playing their console as often as possible, the push for online games and the ever growing list of services prove this. They want your time because they want your mindshare and money. A big title will create a sudden influx of activity much faster than a smaller title but this is likely to dwindle over time and will eventually lead to a lull in activity. This is where DLC and smaller titles take centre stage. I would have to point out though that new titles refresh the urge to play whilst DLC motivates people to return to a title they've already played, and depending on that titles popularity or, more importantly, addictiveness, the results could be somewhat different.
Given that the smaller titles I'm talking about have to also be compelling or at least interesting, I'd have to give the nod to smaller titles. They keep that HDD hot, keep that console on, engage the gamer with a constant stream of different genres and perhaps even broaden the gamers tastes. I think both PS+ and Games With Gold are an indication that console manufacturers recognise this as a very important factor. There are plenty of small titles that come and go without much of an impact, and although many will be loved by some people they couldn't really be brought into the equation on a general basis. Don't get me wrong though, of course they play a role, albeit a smaller role, as of course DLC for less compelling titles do.
Console exposure:
This for me is the most overlooked and yet important aspect of creating buzz for a particular console. Word of mouth is very important when it comes to the negatives and the possitives. It can make or break a game or even a console. I'm sure I really don't have to justify the inclusion of console in that last sentence, we've witnessed first hand the truth of it. But let's concentrate on games as the motive because the past is the past and it doesn't serve the question of 'which is more important' well. The title isn't 'which was the most important'. It feeds into the present situation but I feel it's a different debate and so requires a different thread.
I'm going to frame this next point as a scenario because I want to show that even smaller titles can promote a console and so undeniably must also help sell consoles; not to the degree big hitters do but certainly to a lesser degree. As I've stated I'm a PS4 fan so I'm going to centre this synario around a smaller title on the PS4. Don't consider this bias, just another indication of my ignorance of the smaller titles on other consoles:
Brothers Tom and David own a PS4. Billy doesn't own a console just yet because he's undecided and is still waiting for that big title to get him enthused enough to invest. He doesn't have any allegiance to a particular brand because the concept doesn't even enter his mind. Tom and David invite Billy round to their home. On entering Tom and David's home, he hears them shouting, laughing and generally having a good time. They're both sat cross legged in front of their TV with controllers in hand and turn momentarily to greet their friend before returning to the game. They're both enthralled, exciteable and clearly extremely happy, something Billy finds difficult to ignore because it's a mindset most teenagers long for. He asks them what they're playing and they tell him Rocket League. They don't tell him it's an Indie title or a smaller title, they simply tell him it's 'fun'. Tom stands and offers Billy his place, handing him the controller. Billy begins to play and is soon having as much fun as the two brothers were when he entered. They play for hours, sometimes against each other and sometimes against people online. Just before Billy leaves, Tom asks him if he has a PS4 because if he did have a PS4 they could play online together the following day.
In the scenario above, in weighing up which console to finally buy, Billy is likely to remember the fun he had playing Rocket League. Even if he still waits for that one compelling AAA title, he is likely to consider Rocket League as an extra incentive and so I would have to conclude that the smaller title was in fact a system seller, or to be more precise, a deciding factor for which console he finally buys. The more smaller titles he experiences that are fun or interesting will eventually make that one AAA title he's waiting for an easier decision.
conclusion:
In conclusion I would have to say justification is the most important. A big title is more likely to incentify the purchase of a console but the smaller titles are what will stop people from selling the console, or using it as often as the manufacturer would like. If the gaps between the bigger titles are too large and there's nothing to keep you occupied between them, it's likely your console is going to play second fiddle to football or basketball and your friends are not going to be exposed to the console you bought. They're not going to witness you having fun. All they will see when they come to your house is that your console of choice is always OFF.
The PS5 Exists.
