Splatoon is bundled with Smash with a $50 price drop. and it is selling well. it really might have a chance to beat Halo 5 this year.

if those vgchartz numbers are true (which is admittedly questionable) then it's starting to look possible. Some people are talking about black friday for Halo but the splatoon bundle is huge. Target just said it beat PS4 and XB1 on black friday, not only that but every other item on the website. At walmart they're selling splatoon for $25 and they also have the bundle too. It seems that many people are choosing other shooters besides Halo this time.

bingo 

was waiting for some sense in this thread when scrolling down

digital, bundles and lifetime sales
all that does not favour splatoon.

so i doubt splatoon will win.
not with nx coming next year already and wii u being dead

Habe you thought about competition? Splatoon had exactly zero competition.

Halo 5 has CoD, Battlefront and Fallout. So keeping that in mind it's drop is far from bad and it might do well until christmas.

Splatoon has it's problems as well. It's not doing that good in Europe and it already has a huge attach rate in Japan.

Race to three millions will be interesting though.

I expect Splatoon to reach 3 million by the time 2016 arrives.

Will be very interesting to see... and Splatoon is on a smaller userbase as well

Splatoon is on a smaller userbase, yes. And it's been loosing momentum in the west while it's com to a huge attach rate in Japan, which will be limiting at some point.
Soon every third japanese Wii U owner will jhave Splatoon, if that's not already the case with digital sales.
Right now according to VGC only NSMBU, Mario Kart 8 and Wii Party U are bigger over there, but Splatoon will overtake Wii Party U very soon. So there's the biggest title from console release and Nintendos biggest online title to beat.
And that's where the market shares get interesting. If you look at SSBU or MK8, those games are actually bigger in Europe than in Japan while they're biggest in the US.
Splatoon is big in Japan, not so big in the US and decent but not great in Europe.

Keeping the current ratios Splatoon will have to get to 1.2 million in Japan for 3 million worldwide. With it's lost momentum in the west it's probably even more. That's possible though.
Five million is unlikely though with Wii U already dying a slow death saleswise. Don't expect it to be as longliving as Wii games because most retailers will through out Wii U games far earlier than they do with Wii games.

On the other hand there's Halo. Halo games got legs, no Nintendo legs nut still legs. So it will be keeping on selling until Halo 6. With a bigger and faster growing userbase. That might help sales in the long run but it should help this year as well.

Halo is being bundled as an added extra for holiday bundles, and next year I'm sure we'll see actual Halo bundles aside from the Limited Edition bundle we have now, which is too expensive to convince fans on a budget to buy it. So it's not like Nintendo is doing something to passively sell games that MS isn't. And the mere fact that MS is selling more Xb ones at a base level means bundling Halo 5 with almost every Xb one sold gives Halo 5 the advantage.

Just so you know, the momentum in the west is still above 15k and with the Smash/Splatoon bundle for black friday, pretty sure the game got a huge boost to sales.