To the OP, what miles you have now are insignificant. Once you get decent status, you'll get at least 2 miles for every 1 you fly. Those 20k miles won't mean much of anything. The things to consider in making your choice are being able to choose flights that keep your miles in one place. How often/easily you'll get upgraded and the availability of competetive flights and the lounges you'll have access to are other important factors.
Your best choice is either AS or NW (CO is another good alternative, but the benefits are no better and NW offers many more promotions).
AS will get you upgraded on AS flights. You'll also be able to fly their partners and redeem miles on them as well. Out of Seattle they have some handy direct routes (Chicago, Orlando, Dallas), but they aren't exactly great in being on time. AS also does offer reasonable mileage pricing for points flights domestically. The real drawback is that you'll be flying NW/CO a lot and won't be eligible for upgrades.
NW will let you fly CO, NW, AS and DL. You'll basically be missing out on AA flights, but they aren't that competetive out of SEA anyways. NW offers many mileage promotions and they also upgrade you automatically based on fare class, status and availability. This benefit will apply on any CO flight. Once you get to platinum, you'll be riding up front automatically most of the time. In any case, its effortless and you don't need to do anything for it. You'll also get great availability on NW flights across the lake with your status.
I'm not sure of the exact rules of AS Board Room memberships, but you will be allowed to use them at NW, DL and CO (and maybe more) clubs when travellign on those airlines. An NW membership basically means that you can go into a CO, NW and DL club whenever you want. You can also go into the AS club (very nice on in Seattle).
"Why not join both American & United programs?"
Because they don't give him great upgrade, their flight availability is limited in SEA and he'll end up flying US Air coach quite a bit. Just because you fly these doesn't mean that others should. No need to join two and waste miles. Ever gotten upgraded on United? Didn't think so. It takes being a platinum on AA to get auto upgrades. You get that at silver with the alternatives. Splitting miles up into two groups is wasteful. Those 25-49.9k miles you put into your UA program would be much more if you put them into your AA account. The problem is AA has limited partners which is probably why you have to fly UA.