I'm a MM on CO (IAH based) and switched to DL (after a status match) in 2009. I haven't looked back since. In a hub city like Houston, DL is a welcome alternative - especially if you want upgrades.

I generally liked CO, but after I saw my ug rate continually plummet, I changed. Here are a few thins to consider:

-DL generally has a better ug % for domestic flights
-DL handles IRROPs amazingly better than CO
-Like others have said, it can be difficult to use skymiles for biz int'l flights, but I've found the domestic ones to be relatively easy (esp when you consider the ability to upgrade on a award ticket)
-Dl service in first is a little better (I think). While the meals might not be as good, the old saying still applies "Continental feeds you and Delta gets you drunk"

Good luck with the choice!

FYI - part of the problem with CO (at least here in IAH) is the whole UA thing. I haven't met a FA yet that is happy about it and the CO pilots took out advertisements here against outsourcing jobs in the rj's.