id say there both evenly decent, but they are both extra long, check your case, though id pick the 6800, for quality reasons...
the ati just seems older
and your gonna want atleast 18Amps on the 12v rail of your psu, it should say it on the side of it.
my old 300w, 430 peak thermaltake psu has exactly 18A, but it ran a 7900gt fine