I dunno why, but I've never been interested in going to PAX Prime and i live in Seattle.
I don't know why either

I got to PAX East in Boston every year, it's the best con that I've been to. The problem with Prime is that the venue is WAY too small. A friend who goes to East with me went to Prime last year and she said she would never go again. The Boston Convention and Exhibition Center is way bigger, has a better lay out, and the hotel is attached to it so you aren't walking miles every day.
The cool thing about PAX is that there are so many different things going on at any given time that you can make it what you want it to be. Between the show floor, classic gaming freeplay, tabletop, panels, concerts, and tournaments, you can really make the show yours.