Do you really have to ask? Get Street Fighter IV; Resistance 2 was "meh."
The 360 version of SF4 is technically better, but the difference is really minor overall and it's still a fantastic port. You can't go wrong with whichever platform you get it on.
The upcoming PC version will likely not be as populated online and play as smoothly due to people having different types of set-ups. I plan to get it if I have a good enough PC by then, but no way is that going to be the "lead" platform for the game that people go by.
If you want to wait for the PC SF4, I suggest neither!