Definately superior for maintanance and cleanliness reasons.
A lot of people say that you should have gravel because it feels more natural to the fish but in general I think most fish would prefer better water conditions if it was between the 2.
Some species of fish do not seem to like barebottom at all, especially some bottom dwellers, and I really wouldn't recommend it for them, but most fish seem to take to barebottom as well as substrate.
I would venture to say some fish actually seem to like barebottom better.