Most of the current offers for these cards have language to the effect that you can only get one bonus after July 2011, either UA or CO. Look in the fine print. At that point, it probably doesn't matter. I'd pick the CO card, just in case the limit isn't actually enforced.

(Are the baggage benefits covered for both airlines on either card? Still matters for a little while longer...)

Also note that the last 10k of the bonus is very hard to achieve.