I live in Denver but in central Denver and work near downtown. I have AT&T on my iPhone 5 with LTE and get great signal. But I am getting a Verizon Mini this weekend because:
1) Verizon has better priced data (I am grandfathered unlimited on my iPhone and don't want shared),
2) Verizon offers hotspot on all data plans (AT&T only offers it on their highest iPad plan), and
3) Verizon from what I have heard has much better and more extensive LTE coverage in the mountain corridor and throughout the ski towns.