Have been a CO Elite for many years and was status matched to Gold this year on DL.

I am in agreement with other posters on the pluses and minuses of each. DL has definitely been superior for me in terms of complimentary domestic upgrades. I have actually been upgraded on 100% of my domestic flights with them this year, 10 legs I believe. CO upgrades have been extremely hard to come by, on the other hand.

I have always had good luck with redemptions on CO, but have limited experience with DL. It does seem from the experience of others (and some online searches I have done) that the DL miles are "worth less" (not worthless) so you just need to keep that in mind going in.