Both what?
The point of the level 20 limit is so that different people have different builds of characters that aren't godly by themselves and rely on teamwork.
It's fucking called "co-op" for a reason, 1 person wins, every person wins; XP and money is completely shared, if you can't understand the "co-op" aspect of the co-op MP then you are just a fucking dumbass and you should just go back to playing mindless matches of COD instead.

You can either go 6/6/6/6/0 or 6/6/6/5/3, deal with it.