I guess it would all hinge on what you think about Denver. Going to the Northern lines, you'll be giving up your seniority. If course, if you're new to BNSF, you don't have much seniority to give up anyway. If family, friends and your old job are up there, you might actually be better off. Taking into account that there is still some support when you get laid off.
The boards look pretty full right now, so I don't know what to tell you. At Denver, you still have system wide seniority. That being said, they are recruiting people to move to Denver from the Powder River and they will bring their seniority with them.
It all boils down to if you can ride out the slow times and if you can even arrange to be released to go up there.