Well, I've read the speed on the 360 is going to be faster than the PS3, so it depends if you want a slow paced PES (which the PS3 one is apparently going to be) or a faster paced game. However, the report also said that the 360 final version won't be as fast as the demo, which I've heard is too fast.
The PS2 game won't look as nice, but I'm pretty sure that it will have a more comprehensive editing mode, so if editing original kits is your thing, maybe PS2 is the way to go. Then again, there's that picture import thing you can do on the next-gen game (not too sure about PS3), so you could get more accurate looking kits than creating kits from scratch "manually". Anyway, there will always be hundreds of option files, so if you use them, editing won't be an issue.