In my opinion, they're effectively the same genres. There's some differences, but they're usually aesthetic, not functional. Fantasy is usually swords and stuff, and sci-fi is usually spaceships, but what about Dune?
I think anyone arguing for a hard separation is having the same argument as the "is a hotdog a sandwich" argument. It's fine to discuss for fun, but there's nothing to be gained from it.
Basically, my opinion is to go for it. As long as there isn't an attempt to turn the community into a Lord of the Rings community, I think you're fine posting either, as long as there's a good faith attempt at relating it to the community.