Smell: Definitely onions. Though an asshole doesn't make me cry, poop has never, not ever, come out of an onion.
Taste: Definitely onions. Assholes taste bad. Almost like an asshole.
Look: Definitely onions. They look like they don't smell or taste like assholes.
Mouth Feel: Onions. Pretty confident it's onions.
Sound: Assholes for sure. They play a large role in the post-production of farts. They have last right of refusal on anything that goes out into the world. They're the gatekeeper of the funniest thing the body does.
Pizza Topping: Onions. Unless we're talking prison pizza.
Name-calling: Assholes. You can't just call some asshole an onion. Not only will it not put them in their place, if they're an insightful or even a self-centered asshole, they'll think you're complimenting them on the depth of their personality.
Making Sex To: Man, you dig a good section out of an onion and heat it up in the microwave... just kidding, this one's assholes by a landslide. (Assholes By A Landslide was the name of my band in trade school.)
Short Answer: Onions take it, 5-4! (Though in reality, assholes take it. If you know what I'm saying. Oh, you do? It's obvious? My bad.)