Based on your needs, American is probably your best bet for nonstops. They even have a club in DEN. Although for only 50k a year, you are somewhere in the middle at AA, instead of being on top with Frontier. And with Frontier you get nonstops all over the map, and don't have to connect at ORD or DFW to get to ATL.

I myself am thinking about jumping over to American, but it looks like my travel will drop off this year fairly significantly so it might not matter for me either way. But being MKE based vs DEN based is a huge difference. I think Frontier is a wonderful value and if I were DEN based I would stick with them.