Expect to wait at least a year before Microsoft allows a PC release, possibly longer. It's doubtful that ME 1 will ever be coming to PS3.
There is no official word or even off-the-record announcement of a PC version yet.
The 360 is building up a decent library of games that aren't on the PC, though only a couple of other (Japanese) RPGs so far. If you like FPS and action games the exclusives + Mass Effect might be enough to justify buying one.