I've flown both (in fact, I'm a Platinum level frequent flyer on American), but my choice would be Frontier. They have individual seatback TV's with DirectTV, the cost to use the service is $5 per flight (paid on the plane). Frontier also has some very new Airbus planes that are very nice. The employees at Frontier have always been a pleasure to deal with also, something you can't say about some airlines!
Frontier wins on the Denver hub also. All of their flights are located on the A Concourse, so connections are very easy. At Dallas or Chicago, you could have change in concourses, which can be quite a trip. Dallas has a new Skylink "train" that transports you between concourses which is MUCH better than the old American trAAin.
I fly American a lot, but I'm a huge Frontier fan... you can't go wrong with them.