Be careful before marrying yourself to CO. Although it is far and away the best program for most of us who fly predominantly domestic US routes, its coach product is questionable vis-a-vis seat pitch.
Since upgrades are automatic about 99% of the time in the US, that makes no difference to me, but CO is the most difficult company from which to get mileage upgrades internationally, and operational upgrades are virtually non-existent, thereby consigning you to the tightly packed coach seats almost all of the time.
My guess is that the best program for you would be WorldPerks, where your CO status would be recognized immediately, where the international upgrades are almost always immediately available for 20000 miles RT on a full Y ticket, and where domestic US upgrades flow freely to Elites.