While it's true Primedice has only one game, but they have a really large userbase and have been around for a long time. Betcoin.ag definitely makes a lot of money, but I don't think they make as much as Primedice.
There is no telling on to compare both sites because betcoin.ag status is not on public while we know how many bitcoins were wagered in one day in PD. Also betcoin has spent alot through their pokers with their huge freerolls so I can tell that they could make alot more than what PD earned