Buy a 360 now. The PS3 doesn't come out here until next March (unless you were thinking of importing one) and new systems usually don't get that many great games for a while.
The 360 has a great library of games right now (such as Dead Rising, Splinter Cell DA, Saints Row, PES 6, Test Drive, Condemned, Ghost Recon etc) and some great looking games coming soon (like FEAR, Gears of War, Rainbow Six: Vegas, Lost Planet etc) and don't forget great exclusives in the future such as Halo 3, Forza Motorsport 2, Ninja Gaiden 2 etc.
Plus exclusive titles are getting more & more rare now (both consoles will see Resident Evil 5, GTA IV, Assassins Creed etc at the same time).
By the time PS3 has been launched and starts to get a good library of games together (probably by this time next year) you will know if you want to sell the 360 and get a PS3 so you wont be losing out on anything!
The one area the 360 lacks in (and so did the original xbox) is japanese RPG's so if you are a big fan of these your choice will be limited, but there are some on the way!