I'd grab which one is cheaper, really. I'd say an 1gb 4870 has a slight performance advantage over the XFX XX edition, but if it costs much more over the GTX, I wouldn't take it. Now, if they're priced the same then ATI wins.
Anyway, I'm not a believer in OCed edition of videocards, since you're paying more for a slight overclock that you could just do it yourself, even better then they've done it. So....