Sounds like you've made your mind up with Seattle. Lots of good reasons already mentioned to follow the Mariners. I've been living the same dilema for the past 15+ years, located a couple of hundred miles north of your future location. I think you'll find more Mariner followers than Rockies followers in this part of the world. For me, the choice of which club to follow was mostly influenced by ability to catch a Twins game in person. Obviously, there are a couple of opportunities/year to catch the Twins in Seattle, and only infrequently in Denver. Also, retractable roof in Seattle ensures you will see the game if you travel. Also, it's nice to watch games where you have a high level of interest/knowledge of both teams. The most important thing, however, is that while you can follow the Mariners, you pull for the Twins.