Both QFT!
With random powers one is NOT a warlord, but merely a mobster with whatever they happened to scrounge up for a skirmish.
IMHO, bound armies should be able to select both powers and warlord traits (instant, legitimate reason to remain bound vice going unbound). Logic, and fluff of warlords actually being able to think, use foresight, and make good, tactical decisions, means that being able to pick powers makes sense. Whereas in an unbound army of unlimited codices it would make sense that random people happened to group together on the battlefield, so unbound armies ought to be at the mercy of the random d6.