I've been based at both, and agree with what Electra wrote. My seniority took quite a big hit when I transferred to DEN, but the trips were so much better it was well worth it. That said, go wherever you can hold a line first. That will probably be COS. I think I'd rather sit reserve in COS too, just because it's a lot easier to get through the airport there. DEN is such a pain in the ass that I don't bid for anything less than a 2 day. If I was on reserve there, I'd be ****ed if I got called in to sit ready.