United is no good...unless you fly their codeshare flight on Lufthansa (SEA-FRA)
think about it for a sec, their flights are connecting in Chicago...
why would you want a US connecting? it sucks on the way back because of customs.
you can skip all that hassle with getting a direct flight all the way out to Europe, that way on the way back the only time you'll deal with baggage is at SEA. your last stop.

from what i remember these options all have direct flight out and back
Lufthansa- SEA-FRA-.....
British- SEA-LHR-......
Icelandair- SEA-RKV-.....
Delta/NW- SEA-AMS-.....