A must buy.
PC edition looks great, especially when compared with the low-res PS2 game, but the unmatched power of the PC platform gives it the technical edge. Building your custom radio station is also much easier on the PC, given the Xbox's agonizingly slow CD ripper. This said, the PC version doesn't control nearly as well for the in-car portions, so that's something to keep in mind.
Overall, you can't go wrong no matter what platform you pick. GTA San Andreas is the same great game on any system. This port just polishes an already outstanding experience to nigh-perfection.