Okay, from your post I'm guessing that you don't mind a "parts gun." (i.e. Mismatched upper and lower.)
First, I'd avoid the Colt. You're paying extra money for a prancing pony on the side. In addition, you might have to deal with sear blocks, non-standard parts, funny upper/lower joining holes, and/or a lower that requires a special type of Colt proprietary bolt carrier. Basically, unless you find a green label at a fair price, why bother?
I'd actually recommend a PWA lower. Many are preban, and they are a LOT less expensive. The quality is there; hell, many folks say that they're build like a rock. For the price of a preban Colt lower, you could get a standard preban PWA lower [b]and[/b] a quality upper.
But, as for your question, if you absolutely must have that Colt lower, get yourself a Colt upper. (Just make sure that any sear block/funny holes/blue label issues are adequately addressed with your upper.) If not a Colt, go for a Bushmaster. I have only ever heard one complaint about a Bushmaster upper, and Bushmaster fixed its error immedately and ate shipping costs.
Mike