I don't feel informed enough to answer. I'm familiar with Ravenloft, but don't know anything about Planescape other than it's apparently about traveling around the planes of existence, which seems kind of overly ambitious, doesn't it?
Anyway, I'm voting for Ravenloft because I know what it is, and I could see it being fun.
(Although another part of Forgotten Realms makes *a lot* more sense to me.)
