Ok, Diversity means better play through the single player for a couple goes and the multi player starts off well... then everyone finds the faction and unit that is totally OP and then it is always a rush for the same thing, game broken. Delete... next game.
Balance means a play though the SP once and I have seen it all, but the MP is great fun and lasts a whole lot longer. That way you always have a fair fight. A loss comes down to something you done (or didn't do), rather than a unit you couldn't make. Then that makes a good challenge and will have me playing again.
I think I would go with Balance.