Flagstaff's winter snow is hit or miss, sometimes very dry, and at times they get dumped on. I'm not certain of the above comments, or his/her experience in reference to Flagstaff having terrible year round cycling. Flagstaff has wonderful roads, and one can find a different road or mountain ride every day of the week. My experience having lived there for many years is positive, winter snow doesn't stay on the roads for long due to the arid climate & sun exposure. The single track & forest service roads retain snow for longer because of shade & elevation gain. Winter off season training is excellent, with the Nordic Center, & The Arizona Snow Bowl, you can access cross country & downhill. Also the San Francisco Peaks summit at 12400' which provides great snow shoeing, should the roads have limited cycling access.
Winter training is quite good in Scottsdale, and provides beautiful rolling roads that are cycling friendly. Bartlett Lake has close to 4k' of climbing, Saguaro Lake can also provide close to 3k' of climbing depending on your route. Average temp's in the mid 70's through the winter and a huge cycling community make Scottsdale a gem for winter training. Pez Cycling News did an article on a Scottsdale's Bespoke Inn, a cycling hotel/ winter training destination a couple months ago http://www.pezcyclingnews.com/page/travel?id=124592
The summer is a bit much unless you're on the road by 4:45am, and that's even too hot at times.
Southern Arizona is also very good for winter training, just as the above post indicates, but has similar summer conditions as Scottsdale.