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.