I'll leave my bias at the door for this one.
The PS3 is a more powerful machine for sure, and has a built in Blueray player. Both are strong arguments for it, as so is the free Playstation Network as well. However it currently performs poorly in the game selection and variety Category.
Most of the major releases (CoD4, Orange Box, DMC4, Assassin's Creed) are on both systems. So it comes down to what are you looking for in a gaming system?
The PS3 and 360 both have wireless controllers (perk), the PS3 has 6 axis (see: gimmick), 360 has Gears of War, Halo 3, Mass Effect and Ace Combat 6 (see: pwnage), a Yearly subscription for Live is around $40.00 via a prepaid card if one chooses, and MS likes to nickle and dime users for some of the Downloadable content (see: Highway robbery, Ace Combat 6).
With the exception of MGS and Final Fantasy XIII, most of the real powerhouse games coming out this year are on the 360. As it stands, the only real stand out exclusives on the PS3 are Ratchet and Clank, Uncharted, and Resistance. Lair looks like it has potential, however I am reserving judgement on it.