Heck, I'm also trying to make impromptu decisions on when it's best to use Commander's Strike, and for who. It looks like I should...
Favor Delia when she hasn't used Sneak Attack in the round and has combat advantage
Favor Derren if the opponent's hard to hit (his attack bonus is higher than Baern's).
Favor Baern if the opponent's easy to hit (he does the most damamage on average with a basic attack)
If the others are the only options, Sariel shouldn't use Commander's strike, as she hits harder with a melee basic attack than our wizard even with her Int modifier tacked on, and our cleric needs the bonus from Commander's Strike to do 1 point of damage more than Sariel does on her own... with the same or worse odds of hitting.