I would say to focus on where you can find a job in your field first, and overall quality of life second...you can ride bikes almost anywhere, and almost anywhere is going to offer better trails than central Texas, right? lol. Seattle is tough unless you love wetness and chill...Portland might be a better choice and it's still close to Vancouver (and there are a LOT of great mtb trails near the Portland area, including some excellent freeride zones)...plus all of the hiking, backpacking, skiing, boating, and anything outdoors that you want to do...not far from the ocean, either. Portland is also an extremely bicycle and pedestrian friendly area, with a fantastic public transportation system as well.
Denver is a favorite city of mine, too, and in some ways it might have more to offer with employment. Busier, higher, drier, sunny, with some ok trails in the metro, but millions of trails of all types within a short drive. I think the cost of living in all three areas is pretty similar to what you have in Dallas, but home prices and rent will be higher (homes are way higher in Portland, and of older/lower quality in many parts). If snow matters to you, Denver sometimes gets dumped on, but not as much as one would think, and they know how to deal with it there (the roads are fantastic in the winter). Snow and ice are pretty rare in Portland...it hovers close enough to freezing that it usually just stays as rain, although just a half hour up the Columbian River gorge it may be snowy, and of course the mountains surrounding the city will be in deep snow through the winter.
Several years ago I was considering either Portland or Denver...I was *this* close to going to Portland after several short visits, but ended up staying put. Still not sure if I'd pick it over Denver. Both wonderful cities, just different in their own ways. Can't help with Seattle, sorry.