This would just be my opinion, but DEN has a wider variety of trips and start times. COS has a lot, not all, but a lot of early report times (before 6am) and late arrivals due to being a maintenance base. However, if you live between the two, COS can be more attractive because it is a short walk into the crew lounge from the employee parking lot. DEN is out in BFE and it can take anywhere between 8 minutes and an hour depending on time of day, weather and which route the bus takes to get from the lot to the concourse. COS is a small base which can make for a family-like atmosphere (and everyone knows everyone else's gossip), whereas DEN is about 225 crews and you will go weeks or months after flying with someone before you even see them again. Neither of which is necessarily a bad thing, just that they are sort of two different environments. You will fly to all the same places and on both the 200 and 700 regardless of where you are based. Hope this helps some!