I want to thank everyone who offered me their advice over the past few days. I can be a remarkably insecure, nervous person when I have to make big decisions, and it was immensely reassuring to have people reinforce my gut feeling about the choice I had to make. While I loved Seattle and the University of Washington, I felt like I really couldn't pass up the chance to work with the group at SUNY Buffalo, especially since I would have the opportunity to do work at a volcano observatory and on active mountains.
And, as everyone has said, it's only for two years. I may think that the campus (the North one, anyway) looks like a prison complex and that Buffalo is a dreary former industrial city (which it is at this time of year), but I will also have a wonderful advisor, a great project and, since the cost of living is so much lower in Buffalo, I will not be extremely poor. Everything else is just details.
What really sucks is writing/calling everyone else to let them know I won't be working with them. I'm trying to be as polite as possible, but I still feel like a horrible person. And how the heck do I approach them later on, if I want to work on a PhD at one of those places? "Sorry I didn't choose you the first time around, but hopefully you can ignore that whole rejection thing?"
Oh well. Even if two years go fast, it's still two years. And I suppose they deal with this often anyway...but it doesn't make me any less uncomfortable. At least I can't dither anymore, since today's the deadline and everything is signed, sealed and sent.