Many clubs do a bit of both, with regular singarounds and paid guests interspersed.

Good point. The singaround I go to is completely unlike what I think of as a folk club - no admission charge, no raffle, no nominations by the MC, no facing the front, just a room with people facing into the middle & turn-taking around the room. But it's perfectly possible to have a club where most of those things happen but the club is still recognisably a FC rather than a singaround. I think the big differences are whether there's a charge on the door and whether the MC plays a big role in proceedings or just works his way around the room.