Command squad each and every time! You can give them Terminator honours, get more power weapons, get a standard, apothecary, champion, and then an extra attached character.
Anyone feel like a Librarian with charging into somthing big and nasty with a force weapon, furious charge, terminator honours, and then giving him some Relictors wargear, like the Blade of Phaedron and Shard of the Monolith? That's a S6, I7 close combat monster with five force weapon attacks and then a further D3 on the charge, and coupled with the Chaplain's rerolls is pretty much going to take any large enemy apart.
So, yeah. Command squad. Extra character. And also, as you can give anyone with access to the armoury Terminator Armour, you can actually get six Terminators in there.
Of course, you have to factor in points costs, but... Who cares that the command squad costs something like 1390 max? It's worth it.
So I'd always go for the command squad. In fact, it's pretty rare that I don't use one.
Wargaming should never be combined with your love life or a pot of honey.