"Sounds like you're the one who's biased. United doesn't give complimentary upgrades to GS members, and it's supported upgrade system has allowed me to upgrade 90% of my flights this year. The CR-1s and 500s I earn from my international trips has given me more then enough upgrades to use on my domestic flying. With two west-coast UA hubs, the OP would be much better off going with United. Especially if he does any international flying at all - much easier to upgrade internationally on United then CO."

Of course I'm biased. I flew on that airline for years. I was Premier Executive on UA for 3 years. The latter part of one was in Seattle. The upgrades that you're talking about are coming from a mass amount of overseas travel. The OP is looking at doing west to east coast domestic. Try flying UA just inside of the country and you'll see that the upgrades are garbage. I've flown many a night on UA sitting in coach looking up at the open seats in business class. On CO/NW, you know that when you're flying domestically (as the OP will be doing), the airline is doing what it can to take care of you. When I fly in the back now, I know that the airline I'm flying is doing what it can. I might still be in back, but I know that is because people with more status or a higher fare are sitting up front and there aren't empty seats up there. That's a far cry from UA. The OP will get nothing like 90% upgrades domestically. I was PE and most of mine would end up expiring. When you elect to use the certificates, there is no guarantee of them being able to be used.

There is also the issue of partners. I wonder how the OP will like being limited to UA or US Air. That limits options flying out of SEA vs the CO/NW/DL/AS partnership. With UA, you also have the distinct joy of flying through hubs like SFO and ORD. Both, especially the latter, are known for delays. The biggest improvement in my travel experience has been avoiding ORD. I've been stuck at that airport overnight on at least 1/2 dozen times.

The UA lounges are also terrible. Even the international ones only give you a few drinks. CO/NW/DL/AS all have free drinks and almost all have free wireless. The AS lounge in SEA in particular is quite nice. Both NW and CO also have lounges there. When I had an RCC membership, I stopped even going in to the UA lounge. I didn't see the point.

UA might be a great option for someone who does a ton of international travel, but for anyone flying domestic they aren't.

The best solution out of SEA is either CO/NW for the upgrades on both and the ability to get miles from AS/DL flights or AS for the ability to get upgraded on its flights and get miles from CO/NW/DL/AA flights.