most likely 6s. the reason why 6s works better is because it's easier to find dedicated mains for the 4 classes than it is to find dedicated mains for all 9 classes, a lot of the time you just end up with people that are good at X or Y class but don't really main them, because there's no alternative. and getting 18 people together for scrims/matches is a serious pain in the ass. 6s is better because it's smaller, easier to follow for spectators, and much simpler than highlander.
that's not to say highlander isn't good or whatever, any gamemode where i can play heavy full time is good imo... it's just that it's a lot more inconvenient than 6s