The 360 is already on the market, and the PS3 shall be out some time next year. Most sources are saying Spring... I'm saying June-ish. By the numbers, the Playstation 3 slightly outshines the new Xbox in most regards. It's worth noting however that while the Xbox only has 3 proc. cores, they do run two threads each. Also, it's ironic to note that the 360 is physically smaller than the PS3.
(Care of IGN)
So, I think the choice of games and online playability will make or break these consoles. For your viewing pleasure (care of IGN again), you can now download two trailers for Armored Core 4: one captured from the 360, the other from the PS3.
Scroll down to the first comment and I'll tell you which vid was from which console. It's best to view them with an open mind.