Basically, the one who is younger has the change to grow much more. In that respect, Zouma should be 'better'. But if you want to play one of them from the off, in that case Balanta' s stats recommend him as a more capable first team player.
I have both Balanta and Zouma as my LCD and RCD and they are unbreakable. Its like playing younger version of Puyol and Pique in my team, although Zouma is better than Pique, but Balanta reminds me of younger Puyol.