I find that in most cases, I want full command in my units. The champion to take challenges, leaving the scar-vet (if there is one) to put all his attacks on the unit. The standard bearer for the 1 combat rez that he offers and the musician for swift reforms and tie breakers in combat.
It really depends on what you want to use them for. At the very least I would add a musician for swift reforms, standard bearers are great for the +1 CR and for adding various magic standards (I like the flaming banner on mine for hydra/abom hunting), and champs give you more attacks per frontage and let you challenge nasty characters.
Hello, I recently made 5 CoC that are in the batalion(It was either that or finish my masters dyploma And When i looked on the cost of the full command I was shocked. I would say go with the musician only, because CoC is way to expensive and too slow to do some real damage, but if you have a flank charge with them esp vs ogres they can be of use, but a sandard and champion are way to expensive for me.
I agree with Vapor and Arli that if you take a Character with the unit you should take the full command. If the unit is not going to have a character I wouldn't spend 20 pts on the Champion for one extra attack. Although it's worth noting that if you're running your Calv into units with out support then the champion is nice to soak up a lot of the extra damage from a powerful hero. Champion is basically a 20 pts damage sponge for your unit.