Generally speaking, it is much easier to get advanced confirmed upgrades on DL. One of the nice things about DL.com is that it displays F UG availability on all flights in your elite status UG window. I usually book all of my DL flights five days out and select the combination of fare and instant UG availability. Just booked a $190 OAK-SLC-DTW for later this week and was instantly upgraded. These days on CO, you usually don't get the UGs until the gate... not very reassuring.

On the other, as noted by others, good luck trying to redeem your DL miles.