Probably a very personal decision. I switched last year from UA to AC. I liked AC's lower mileage requirements for awards. I never did the whole upgrades thing but have always just booked a full award instead. If you plan on doing upgrades I think UA is a better choice.

Another reason for my switch was that I had an Amex Gold membership rewards card that came with a whopping 50K points sign up bonus. Those points were 1:1 convertible to AC but only 1:2 convertible to UA. Then I dumped that card after moving all the points to AC and got an SPG Amex card with a 10K mile signup bonus and a further 15K bonus for spending $15K on the card in the first 6 months. And I got the BoA Aeroplan MC which had a 15K signup bonus I think. I ended up with 120K miles in no time due to all of those bonuses which was enough for a F/C *A award on LH/TG/NH to SE Asia via Aeroplan. I believe the same award would have cost me at least 145K with the UA program.