As a CO* and PLT I find that upgrades are a strange animal with CO, if you buy higher fares you have a far higher chance of success than lower fares.

I do not know if either UA or AA offer this perk but if you buy "Y" fares you automatically recieve a upgrade to FC at time of purchase. The upgrade comes from CO's "A" bucket(which is CO's bucket for full fare first class), thus it always clears unless F is full a time of purchase.

Otherwise, I would say that if you are happy with UA, stick with them, as others have said it is too hard to know which airlines are going to survive and which are going to fail.

Dan