I feel like the question of "is this sci-fi or a fantasy set in space?" is very different from the question of "which category should I put it in?"
Like, Star Wars is absolutely a "fantasy" story with lasers and robots. Mystic monk and cocky rogue become friends, save a princess, and help badass princess defeat the evil dark lord. But if Star Wars were a comic that was joining Tapastic, I would tell its author to upload it under sci-fi, because that seems like the audience that'd be more receptive to it.
I feel like the only conceivable time your genre will matter is when someone is looking for a specific genre, and they'll generally be expecting to see the trappings of that genre. Someone who specifically enjoys fantasy and goes looking for it is probably not going to click on Star Wars, because it looks like space ships and robots. Someone who enjoys sci-fi and goes looking for it (in comics especially) will probably give Star Wars a chance, because it's in a setting they enjoy.
It may actually be a dungeon crawler fantasy set in space, but if it LOOKS like a sci-fi, it might make more sense to call it sci-fi. I think calling it fantasy would only really make sense if it doesn't really look like sci-fi -- that's my feelings, anyway, but if someone has more experience with that genre on Tapastic, please listen to them instead!!