The idea of putting all your miles on Alaska is an interesting one, as it combines your flying into one account. However, it does not get you as many upgrades.

I'd suggest dropping back to SILVER on Northwest and getting up to 100K on AA or UA. When I was CO-Silver, I got about 95% upgrade on NW. It depends on your routes, but I would say the odds are just about as good when Silver as when Gold.

Use NW to go to MSP and other cities where you have to do one stop anyway. But, shuffle the rest of your travel around so you focus on AA or UA to get to the 1K level. Then try that for a year and see if the benefits are really worth it to you or not.

If your primary goal is domestic upgrades, NW is the way to go. If, however, you think you are going to do some int'l for business, AA/UA have their advantages. Also, if you want to redeem your miles for an int'l trip, AA/UA will give you some better options and it will be easier to get a seat for miles.