I had a PS3 then got an Xbox 360 later on so I kinda know what you're going through, in tryin to pick out stuff you haven't already played.
My games are PGR4, Gears of War, Takes of Vesperia, Halo 3, Mass, Effect, Lost Odyssey and Ninja Gaiden 2.
Let's just run through this for a minute.
First off, you got Halo 3, which is probably the best Xbox 360 exclusive. This should keep you happy for a while. I loved GTA4 too but I'm gonna say.. meh, fuck it to the DLC. I mean it's your call but these open world games don't entice me with DLC. I'm not saying some expansions aren't great, but with the open world games like Fallout and GTA I have trouble paying money for the download, let alone the idea of buying the entire game again. I feel like you'd be better off getting an entirely new game with the same money, but that's just me.
But let's talk about exclusives.. well Tales of Vesperia and Ninja Gaiden 2 are both going to come to PS3 with superior versions, though you might find some people here debating over NG2. Read about the differences and decide for yourself, it's a game worth owning for either console. ToV... just wait. It's a great game, get the best version.
Gears of War is awesome. It's short, the multiplayer is worthless but the campaign co-op is a lot of fun and it looks great. GoW1 is cheap enough at this point to be an easy pickup. You can use that to gauge if you want GoW2 for its price.
Mass Effect I just didn't really like. I was so interested in it that I bought the Xbox but it's easily my least favorite of the 360 games I own.
Lost Odyssey is a great game but if you've played FFX as many times as I have, you'll feel like you've played the game before. Nonetheless, it's a great JRPG for Xbox 360 and it's probably the most traditional of all the JRPGs that are out this gen(not counting Blue Dragon cause it sucks).
N+ and Braid are the best XBLA exclusives I think. Braid's a little pricey for what it is, but it's an amazing design.
Also don't forget there are some Xbox 1 games you can play. Halo 1/2... Fable 1, kotor 1/2, elder scrolls 3 and panzer dragoon I've got. I've only had time to play Halo 1 and Fable, both are pretty good, though Halo feels and looks very dated - I wish I played it when it was new. Fable actually holds up better.
I've also been thinking about getting Dead Rising. It looks fairly simple but it's cheap and I can see myself just playing and playing it and enjoying myself if I smoked a lil herb.
Hmmm... what else? Just be careful cause the Xbox community is a lot more enthusiastic than Playstation, so just about every exclusive is always called amazing. There are so good games out there though, but it seems... just like PS3, the great multiplatforms outnumber the great exclusives... like oblivian, the orange box, gta4, cod4, ut3, fallout 3, burnout paradise, assassin's creed and eternal sonata..
Left4dead I haven't bought yet, though I'm pretty sure it's awesome based upon the opinion of people I usually fall in line with. Hopefully I can put a dent into my backlog and then pick it up.
Oh yea, avoid the 360 version of The Last Remnant. Infinite Undiscovery deserves a little investigation I feel.