@Yellowrod Rentmen is more of the premiere/higher-end site listing escorts. I don't know for sure but I believe it's more expensive to post your ads there, and they have a more formal review & moderation policies. They have membership levels and credit card processing and hosted chat and a mobile app. Basically they're more mature of a business and more legit (though some disagree).
Friendboy.pro is more casual though it seems to be getting more traction. I assume it costs less (maybe even has free options?). They don't have all the extras.
Then there's Mintboys.com which is sorta the least formal, and I believe an escort can advertise for free.
That's how I would categorize it, though this is all just anecdotal.