I'm surprised that someone who flies this much is asking this question. Putting miles into a different program will cost you...a lot. You likely won't be flying enough miles on the other carrier to get the status you need for the 100% miles match. That means, for every mile you fly, you'll lose 1 mile you would have gotten with United. You could probably status match to avoid this issue for the first year, but in the second, you'll probably lose the extra miles.

With that said, if you insist on doing this, join Alaska's program. You can fly AS, DL, NW, AA and CO and still get miles on the program. They also partner with a ton of programs internationally such as BA, Cathay Pacific, Air France, Korean Air, Qantas, etc. They have a direct flight from BOS to their SEA hub. To boot, if you fly on their metal, you only need 40k miles to hit MVP gold.