I would pay more for US Airways (which is in the process of combining with American Airlines).
The good experiences that people had with Frontier 2 or 3 years ago are not applicable to today's Frontier Airlines. At the end of 2013, Frontier Airlines was acquired by a private investment group led by former Spirit Airlines chairman William Franke. Over the course of 2014, Franke applied the formula that worked at Spirit.
The price difference may not be as big as it appears when you look at the total price.
US Airways and Frontier both allow each passenger to take one carry-on (overhead) bag and one personal (under-seat) item. On US Airways, the carry-on is free. On Frontier, the carry-on bag fee is $30.
Both airlines offer advance seat assignment. US Airways only charges for "choice" seats toward the front of the economy cabin, but Frontier charges its economy passengers for all advance seat assignments.
However, the biggest difference for me is what happens if there's a schedule change or a weather (or mechanical) cancellation. US Airways has a huge network of flights, especially combined with American, so the airline is well-positioned to get passengers to their destinations. US Airways even has interline agreements with the other legacy carriers and will accommodate passengers on another airline if there's a mechanical issue. For Frontier passengers, it's a another story (as some of the posts in this thread indicate).