I fly a lot US - LatAm flights and switched from AA (Primary) - Aeromexico/Delta (secondary) to UA (Primary) - Aeromexico/Delta (Secondary) because AA was so unreliable. Connecting in IAH is nice and Copa is a huge bonus. Pick United in my opinion.

Don't underestimate Copa for within LatAm travel. It is usually the most efficient way to connect between two mid-tier cities, particularly if going from North-of-Panama to South-of-Panama. If your future might hold a Mexico City - Barranquilla, Guatemala City to Rio de Janeiro, or even Bogota to Lima flight, it will be possible with a simple, hassle-free connection and you'll be able to upgrade using PlusPoints. I think I agree with you that Avianca is not that valuable for your case, but Copa is a huge plus.