As a CO silver elite, I have received upgrades on both CO and NW, and I must say, flying First really makes a big difference, particularly on CO. I have found CO to provide consistently good service up front, from the quality of the food to the friendly and attentive FAs. I usually fly IAH-EWR and back and try to book 777s and 767s with the BF seats. It really makes flying a much more pleasant experience. NW is also a fairly good product and I'll often fly them due to lower fares and easier upgrades, but their F product is slighty inferior to that of CO.