It's based on your Hero vs Bitch score for Bareshade.
If your Hero score is higher, you will top him, if your Bitch score is higher, he will top you. Not sure how it works if they're equal.
Oh, and if you need to lower your Bitch score (the Hero score only goes so high at the moment), if you've already become a regular bitch for the caravan guard(s) at the track, go to the track, take a break until the guard tries to get you to suck him off, but tell him no because there's too many people around. That lowers your Bitch score, and from my experience is repeatable. If you haven't yet become a regular bitch for the guard, you can take the steps at the Track to do that since it will unlock the method for lowering you Bitch score, it won't matter that it will raise it in the process.