There is also AX - Aces which for me is the best of the three airlines in terms of service, staffing and punctuality. I travel this route about 10 - 15 times per year and know it pretty well. Problem with AX is the 30 minute stop in MDE Medellin, but it has advantages. It arrives later. AV and AA leave at the same time, very early in the morning, so the airport is a mess. AX also arrive to the smaller terminal in MIA later, so there is considerably less congestion in inmigration, less time standing in line. Also love their airbuses, and the sitting pitch is quite good (although I always choose emergency exit - row 11 - row 10 the sits dont decline).
I -NEVER- fly Avianca. It is by far the largest airline in Colombia, with many more daily flight locally than the other carriers together. Sam is owned by Avianca so the same goes here. Avianca has continually surprised me for its lack of ethics and respect for the customer. 2 flight three hours appart half full? Well, lets cancel the first! Etc etc. The staff is outright mean to the costumer. My hate for this company goes to the point that I change my business schedule around to avoid flying them - or I will do a sleep over if I cant move my appointments.
You would -not- believe the stories about this company. It is broke, it is highly burocratic (highest employee/pax ratio in the world after aeroflot, I believe I read) and highly monopolitic. It has absolutly no respect for the customer. It is a company formed in a time where, by law, it was the only carrier in the country. Thus, its attitude.