Both are actually pretty hot, and the guys I bottom for enjoy both. I generally prefer to take a load in my ass, but they're just different experiences for me. Sucking a cock well take some work, to move my mouth and tongue all the way down his shaft, and keep the suction just right, and respond to his moaning with what I've discovered he likes. I like how much control I have over his orgasm, and when he sprays my tonsils, it's a more visceral pleasure for me, since I experience it more closely. Plus, the taste of semen stays in my mouth hours later. Unless I'm riding a guy, being fucked is often about submission, spreading my legs and letting him pound and drive his semen in me. I love to suck a guy first to get him hard, and then let him cum in my ass, usually.