Better to do it before hand, so that people who don't play on whatever console can plan to not come, or not enter the tournament. And no one suggest running both, cause when someone who won't play on one console has to play someone who won't play on the other, someone ain't going to be happy.
DJ is funding this free tournament, so it's really his call. And since this tournament came about so he can get practice for majors I'm pretty sure this is going to be a PS3 tournament. 2 frame native input lag and all.
Get hype!