Personally I prefer the multiplayer deathmatches on GRAW to GoW but both are incredibly solid games.
Awesome news Koopa. Very brief run down of some of the 360 games you don't hace:
Gears of War: Very good game. Got that Halo factor of a not huge single player but which you replay over and over. Great gameplay. I found it suitably different to GRAW to be a worthy purchase. Very dark. Great plot.
Amped 3: Awesome fun game. So much easier than old Ampeds. Tons of pretty style and funny cut scenes. Great, varied soundtrack. Very cheap and a great pick up and play game.
Ninety Nine Nights: Not worth full retail price but if you can find it cheap it is very fun. Got a great Lord of the Rings vibe to it. Good plot. Fun and mindless hack and slash with replayability. A spiritual successor to Guardian Heroes.
Quake 4 (+Quake 2): Fast, mental action. Cool style. One for fans of the series. Slowdown is not a problem like some would say. Plus it has Quake 2 too. I like it but try the demo and see as it depends on your tastes.
Perfect Dark Zero: Appreciated by most critics. A great, over the top and brash game. Great single player and decent enough multiplayer. Very colourful with James Bond esque locations and campy acting. A lot of diversity. Think Timesplitters but not rubbish.
Kameo: Beautiful game but short. Now has online play though. Not a platformer though. It's a beat em up like Streets of Rage.
Lumines Live: Expensive compared to other arcade game but still half the price of Lumines 2. You HAVE to buy the Advanced pack if you get it though otherwise it isn't worth it. The most addictive game I have played on 360 and one of my favourites.
Condemned: If you like horror/scary games and can handle a first person view this is a must buy.
Dead Rising: HUGE adventure game. Rock hard at first, easier later on. The final 12 game hours in main mode are a right laugh. Not scary though it is gory. Lots of humour. Tons of hours in it. Constantly surprising.
Tomb Raider: Amazing game. Brilliant reimagining of the series. Even better than the first one. Controls so fluidly. Definitely recommended.
Ridge Racer 6: Great arcade racer. Similar in some ways to Burnout but with a lot more emphasis on avoidance. Personally I prefer it but if you have Burnout it is possibly not worth getting.