Both!
But if I had to pick, I'd say tits... and when I say tits, I mean big natural ones. Mother nature is not too kind in giving big natural boobs out.
Yes, you can get a boob job but they are not the same. I know some "ass guy" will say that not all big natural tits are sexy and that is true. But there are many that are.
Ass is great too!!! But realistically, most fit girls can work on improving their ass naturally (ie running, squats and other gluteus workouts). Can't do that with boobs.
On a side note, other than physical beauty, you have to appreciate the sexual function of both boobs and ass. Any girl can ass fuck, big ass or not, but not every girl can titty fuck.