Ass for sure, there's nothing sexier than a hot man with a tight firm ass, god I can just grab onto it all day! Even if you don't have abs, if you're skinny, toned and swimmer-built, that's hot too! It's such a turn off to see a hot naked guy with a saggy weird butt !
Just like the pecs versus abs question I'll still go with abs. Although a guy with great abs with a really dud ass is more disappointing than one with great abs and no pecs. Additionally, for me, truly great asses are fairly rare while great abs are easier to find.