NW - definitely. They have the easiest earning opportunities. I don't recall which promo it was, but my roommates & I all earned 10K for doing a 10-bar (iDine) bar crawl in DC & taking a $130 RT flight to PDX the next morning - NW is always doing promos & their redemption availability is pretty easy to work with. However, with CO, you can always transfer the miles in to Amtrak at upto 25K/yr then over to UA and use UA awards. I recommend NW unless you're looking to filter your miles into Amtrak to United.

Editted to add - if you transfer over, you've got to do it in 5K increments upto 25K/yr.

[This message has been edited by bhatnasx (edited Feb 24, 2004).]