I wouldn't waste big money to go to the Tacoma Dome unless you have floor seats in the front sections. I went to the show there last year and in 2009 and unless you were up front the sound is pretty muddy. But, I think that's typical of a big arena. I'm not sure how much better the sound is in Portland, but at least the Rose Garden doesn't feel like it's going to come down on you.
I don't know how much help this is, but thought I'd at least comment. Good luck!