I think Sam is better, Dean seems to be too emotional and creates more problems then he solves. Granted both have opposing views on things, however Sam is calmer and less fly by the seat of your pants.
I like Sam's character more, I feel Dean is little of an obnoxious hypocrite at times and I dislike his "tough boy" character cliche and I feel it doesn't allow his character progress anywhere throughout the story.
But frankly the only character in the show I like is Crowley.
Though I love them both equally, I'd have to say that I've always had a slight preference for Dean. It has nothing to do with physical appearance, but Dean has always been the more rational of the two and would've given everything to protect his brother.
Dean by a long shot. Sam is cute but that's basically it. He's cute. Dean, on the other hand... Dean is EVERYTHING. Hot but also has his cute moments. Awesome personality, funny, sarcastic. Tough but shows his vulnerable side on an occasion. Entertaining as fuck. High quality character.