What games are you into because each console has it's own like.. Territory. An avid Rpg player then your choice is the Xbox 360 hands down. Just this year their have been more Rpgs released for it then Playstation 3 has all up. If your more the action guy then either one of them would be a good choice as both have a good selection.
Also gotta think about lasting appeal such as DLC. Xbox 360 has more, also Achievements for all it's games. Only some for the Playstation 3 but that will change with time. I'll just bring up a list of games out and up coming for each console..
-Xbox 360-
Gears of War - Great 3rd person shooter with some pretty good online. Excellent co-op.
Kameo Elements of Power - 3rd person adventure/platformer-ish game that is very well done and unique.
Lost Odyssey - Just a great Rpg, Four discs long and has some well worth moments.
Eternal Sonata - I haven't played this but hear great things about it. Beautiful graphical style to it and apparently based around music.
Halo 3 - Although overhyped really badly still a good game to play. Has a great online and a lot of little extras that make this game overall a good purchase.
Crackdown - Like Grand Theft Auto on crack. Plus your a cop. Great fun gameplay but story and missions are pretty much the same all the way through. Fun co-op and DLC such as Keys to the city give it a bit more lasting appeal.
Mass Effect - Although a bit buggy still a worth while game to consider. Alien sex scene if that's your thing haha.
Command and Conquer series - Great strategy future war game that works amazing on the 360.
Dead Rising - Run around grabbing anything you can use as a weapon in a mall full of zombies.. With a twist.
Forza 2/Test Drive Unlimited - These over Granturismo anyday.
Ninja Gaiden 2 - I haven't played it yet but I hear it's really good.. And challenging.
Plus backwards compatibility with Xbox One games. Plenty of arcade classics as well as stuff like Braid.
Upcoming games..
Fable 2, Gears of War 2, The Last Remnant timed exclusive, Banjo Kazooie, etc.
-Playstation 3-
Metal Gear Solid 4 Guns of the Patriots - Not much into this series but you may like it. Be warned over 30 minute cut scenes.
Uncharted - Fucken awesome game, creators of Jak and Crash Bandicoot. Funny and almost feels like a younger Indiana Jones.
Resistance Fall of Man - One of the best FPS's Ive played. Definitely worth a play.
Heavenly Sword - Very short and not really worth full price.. But it's pretty well done and worth a rental at least.
Ratchet and Clank FTOD - Great game if your into platformers or the Ratchet series in general.
Warhawk - Just fucken fun online war. You get a choice of planes, ground, vehicles all with 16+ online matches. Downside is it has no single player campaign.
Motorstorm - I hate this game but you may find it alright.
Granturismo 5 Prologue - If you've played the 4th you've already played this. Amazing visuals though.
Ninja Gaiden Sigma - Just a port of Ninja Gaiden from the Xbox one but with a few added extras and side story.
Upcoming games..
Resistance 2, Littlebigplanet, Infamous, Killzone 2, Tekken 6, White Knight Story, God of War 3, etc.