There's also one at the Pima Air and Space museum, near Tucson, AZ. Considering that and a minuteman silo are the only things worth seeing within 100 miles of Tucson (kidding, but not by much, the "Landmarks of Tucson" article on wikipedia is 10 entries long, with 3 entries actually in Tucson), I'd recommend keeping it for when you need to go to this hellhole, and doing more interesting things in or around Seattle.
http://www.museumofflight.org/aircraft/lockheed-m-21-blackbi...
If you're in Seattle and have an opportunity, visit the Museum's restoration facility at Paine Field in Everett (at the opposite end of the runway for the 747/widebody factory.) It's a nondescript warehouse manned mostly by super kind volunteer retirees. Unlike the roped-off museum experience, at the restoration facility you can touch and pick up things. The last time I was there I held a Merlin piston in my hands. So cool.