Better snow and demand in Colorado.
Breckenridge would be a good start as a rental property because it has a lot of winter and summer traffic. That being said, if you're there full time in the summer, there will be a lot tourists around as well. It's a high elevation so cooler summer temps.
I also like Crested Butte. Mt. Crested Butte will be better for the winter ski crowd, but not as much summer demand. If you go in the actual town of Crested Butte then you'll still rent well in the winter, but you'll probably enjoy your summers more. It is a great little town and has a more local feel than Breck.
I just got back from several days in the Frisco/Dillon/Silverthorn area. This would be a nice place to summer because of Lake Dillon and all the surrounding activities. There is no skiing on site, but it's on I-70 and within 20 minutes of Keystone, Arapaho Basin, Breckenridge, Copper Mountain, and Vail.
I really like Gunnison during the summer, but I'm not sure how well the winter rental would do because it's 25 minutes to the nearest ski area, Crested Butte.