for most of the games it doesn't really matter. The graphics/performance differences are pretty small.
Personally I prefer the xbox 360 controller so I would get all of them for the 360 if I still had mine.
If you prefer the ps3 controller, get them for the ps3.
The only game you might need to worry about is Skyrim. Fallout games and Oblivion have been notoriously glitchy on both systems. But the ps3 versions have always been the worst. There is no telling if bethesda fixed this with skyrim since no one has played the games yet, but the safe bet would be to get it for the 360.

DeUS eX:Human Revolution - Identical
Batman: Arkham City - 360 has 2xmsaa (less jaggy)
Dark Souls - PS3 has a consistent frame rate and it's less jaggy
Dead Island - PS3 has better textures but 360 has a more consistent frame rate and no screen tearing.
Skyrim - Not sure but probably 360.
Dragon Age - Not sure.
Battlefield3 - PS3 has a better lighting solution and better anti aliasing (MLAA)

All these games are pretty much the same on both platforms, these are small differences.

DeUS eX:Human Revolution - Its the same game
Batman: Arkham City - 360 has 2xmsaa (less jaggy)
Dark Souls - PS3 has bettre frame rate + More people to play online.
Dead Island - PS3 has better textures but 360 has a more consistent frame rate and no screen tearing.
Skyrim - Get for 360.
Dragon Age - 1 disc on PS3. Xbox is in 2 discs. Bettle frame rate on PS3.
Battlefield 3 - get it for PS3. Better overall

All on PS3 except Dead Island and Skyrim.

You are talking about Free online....Then you should get the games with multiplayer for sure for PS3 (something users above me don't tate into account) if you have Gold then you should get them for X360 because those games will have an bigger online userbase. (except for Dark Souls).

My philosophy is, if you have Live Gold, get everything for Xbox unless it's exclusive or there's exclusive DLC or a big discount.

But really, flip a coin. Unless you put them side by side, you can't tell the game difference--but you already know this.

- Deus Ex: Doesn't matter really so if you're trying to grow your PS3 library get it on that
- Arkham City: Doesn't matter really so if you're trying to grow your PS3 library get it on that
- Dark Souls: PS3 by a whisker of performance.
- Dead Island: 360.
- Skyrim: 360 although really you'd be better getting it on PC
- Dragon Age. PS3 is a tab better looking
- Battlefield3. PC or nothing from the sounds of it. If you have to go console PS3 seems to have slight edge

All on PS3 as they are all about the same overall but PS3's online is free, it has trophy's=achievements so I don't see why that matters, its on bluray which helps on BF3 install,...

Really the only reason I choose between them is known issues (people stated some above) and for the games with strong multiplayer (BF3) where the most of your friends you want to play with are playing that game.

Deus Ex on pc, 3 sec loading times really help while going for all stealth.
Skyrim on pc for the mods and messing about with the debug commands and config files.

If you have a really good surround system go for the ps3 for the rest.
If you really can't stand screen tearing you're probably better off with the 360.