Short stories, definitely. The only thing is, it's a lot harder to write a good short story. With novels, there's more room for development, and even if some of it ends up as filler, it can still be more satisfying than a poorly written short story which ends up seemingly lacking.
This is a male perspective now: typically with erotica I just want something straight and to the point, so short stories fit that more. I'm not willing to invest in an erotic novel, unless its focus is more on a great storyline with some hot sex scenes added in.
So short stories for me!