If you can only afford to have one system and are primarily interested in using it for games (i.e., not movies), I would advise you to sell the PS3 and get the 360. I own both systems and the only games I can honestly say that, while the PS3 is a competent system (it just won't have the software yet to prove its true capabilities), the 360 is a better all around gaming machine - especially when it comes to online gaming. True, there will be PS3 exclusive games that you'd miss out on (e.g., Heavenly Sword, possibly Metal Gear 4), but those would be offset by games like Halo 3 or Gears of War. I only purchased a PS3 because I wanted to add a relatively inexpensive Blu-Ray player to my home theater rig. To that end, it's great. However, I don't imagine I'll be gaming on it much until some heavy hitting exclusive games come to market.
Good luck in your decision!