Hmmmm a couple things I can think of...

I think the Bellevue - I-90 route may be a bit more hill and traffic friendly than the west side. The I-90 route also eliminates the need to get through downtown, which while not impossible or dangerous can be really slow...
The Burke starts to wear on most people who use it often because its bumpy, has a lot of cross streets and can be crowded in nicer weather. Of course you can avoid most of the BG and cut some of the milage by going through Lake City (it does mean climbing a big hill though) The basic route is Lake City way to 35th to Ravenna which turns into 25th, but if it interests you I can be much more specific.

If you are up on the hill off Juanita Drive then there's no avoiding it, but if you are just in the area, then Waynita is an easier climb.

It won't work for the Xtracycle, but with a regular length bike you can take a bus across 520 if you are in a hurry. It can be a challenge to get a spot at rush hour though as most of the buses have 2, 3 at most rack spots. Metro is giving free lifts on deadhead buses from only the last stop before to the first stop after the bridge. I think they may even be allowing bikes on the bus to fit more riders too, but as far as I know there isn't exactly a regular schedule so it can be a bit dicey to wait there.