I've used them both and found rbenv's Just Works factor to be higher for me, personally. There's definitely enough people out there using both tools that neither can be said to be the one that "everybody is already using".
:) I mean "everybody is already using" in the context of most dev area, including my own.
Here's how it usually works: We're coasting along with some tool and doing well, then some dev announces that we're ALL WRONG because another tool is better. Sometimes the tool he's announcing is something like git when we're using TFS, and we switch. Other times, say RVM vs. this right now... I just don't see a big gain.
I hear you. I never had any "real" issues with RVM other than I probably never really learned how to use it, so it kept tripping me up. (I'm not very good at this stuff..)
I was setting up a new computer a few months ago and decided to give rbenv a try and it hasn't given me any reason not to like it so far. Just guided someone on the west coast who has never worked with Ruby before through installing it and getting her first Rails project up and running last night, via approximately 65 emails. Does that make me an advocate?