The biggest thing you have to watch with CO is deeply discounted fares only count 50% EQM and no EQM segments. I overlooked this twice in 2007 which then necessitated MR in December for $225, although my late booking Dec 06/Jan07 to LAS for $700+ had me scratching my head on that one (didn't mind the fare because I had $500 bump voucher). It all depends upon your travel patterns. I book cheap because it's my nickel. If you think you are going to be close and you book cheap, stick with NW. Once you get elite you can then be in good shape if/when you need to fly CO.