The nice thing about Buffy was that she was also a slayer and therefore had all the same powers Faith had. She was just a strong as Faith, if not stronger (she had been at it for longer, but on the other hand Faith was arguably more aggressive so maybe it was a toss up).
With Buffy she could let go in a way that she couldn’t with other people. Well, maybe she could but the authorities might have ended up having to get involved later down the road if she tried, and she didn’t want that. She could leave bruises on Buffy and Buffy could bruise her.
So yeah, being with Buffy was a lot of fun. But after it was all done and they laid there not talking, Faith sure as hell wasn’t going to let herself fall asleep. She knew for a fact that Buffy was only pretending to be unconscious anyway.