Delta for domestic travel and free upgrades (even on awards!). CO for *A and easier intl and domestic awards.
DL also allows a free stopover on award tickets, which is huge. Just that you have to expect to pay 40K or more miles for a round trip domestic award ticket. Very hard to find 25K awards for DL domestic flights. In fact, nearly impossible unless you are extremely flexible with dates/routings/even O/D.