Since you are already leaning toward Skyteam due to fare availability and fly in/out of MEM, I think NW is your best bet. Upgrades are more likely to be had than with CO, and as already stated, you'll receive 100% EQM from all Delta fares and 50% or 100% on CO. Also, if you ever travel with another person, the platinum companion upgrade is a nice unique benefit with NW (that's at the 5 day out window, not just at the gate).