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You can expect that later this year CO - probably late Q3 or in Q4 - will no longer be a NW partner. I don't really like the DL SkyMiles program at all, but if that is what is convenient for you, there is likely value in that. I do feel that you'd be better served at your travel levels focusing on a single program rather than splitting the credits, but I can't make the decision for you on which one that should be.