Sorry for double post, but seeing the new GOT episode makes me want the north of the western empires to band together for the sake of the north :P
It'll probably be easier to decide in the Camp Phase. There you will get an idea of who you'll be facing. You'll want to formulate your strategy around that.
EDIT: What I mean to say is, some of you may feel more/less confident about the role depending on who you'll be facing. If your unit is an integral part of the proposed strategy, I would probably want to be Commander. If you think you have a good idea on how to win the next battle, I would probably want to be Commander.