I never use either on my team and never will, but Bellossom is my favorite between the two. Generally speaking, I don't like poison types. Which automatically means I dislike Vileplume. Vileplume's better Sp. Atk is good, but other than that, it has nothing going for it in my opinion. Bellossom looks better, isn't a poison type, and dances or whatever. I always liked the way Bellossom looked, so, it would be my favorite between the two.