i think he was asking about sli vs crossfire, rather then their cards. Supposed ati's crossfire is suppose to "pwn sli like an uber nub t0 t3h m4x0r5!!1one!" according to my other computer nerd friends. Chances are it will be at least a little better, as they are not rather the pioneer, but can use things that sli developed and lightly base theirs on that, so it should be better. It is also suppose to be more stable, and where as if you use two 7800 gtx, you don't get the peformance of two 7800 gtx's, you get the peformance of like 1.6 7800 gtx's. ATI's crossfire supposedly to give a bigger peformance increase by using two cards.
PS: before you nvidia fans jump down my throat, im an nvidia fan myself, and most likly the r580 will beat the 7800 gtx, and most likely ati's crossifre will peform better then nvidias sli, but then nvidia will release something to top that, it just goes back and forth, and soon ati will have the best card (for a little while) out on the market, and the best option for dual gfx cards.