For mileage accrual DL is better. For domestic upgrades DL is better
For Intl travel UA/CO is better due to a better alliance.
If you have some patience, DL miles are quite valuable, for simpler tasks go with *A.
Interestingly I have burnt more DL miles in past 2 years; before that it was all *A.
Since you will fly Intl on awards mainly, there are arguments for both, but favors *A. The gripe about DL is that a paid YBM fare is needed to upgrade, while on UA you can go to the next 3 levels for a SWU and CO now has an any fare SWU. You can not get them with your flying pattern of 25-50k, but others may sponsor you perhaps, while on DL they can not.
You can boost your status on DL to PM which will give you better benefits and wider award selection as well.