Upgrades are also my priority when picking a FF. I also have been able to book premium economy but not business and am happy to spend miles upgrading.

I switched to BA not long ago because of their premium economy andthe ability to upgrade for a reasonable number of miles. The availability is an issue but it's worse on some routes and than others. You may find that choosing different connection points might change the picture substantially. The situation may also change as currently upgrades come out of business award buckets and when a flight is full they may want people to upgrade but not want to allow additional people to book -- and there are rumours BA is working to change this and separate the two buckets.

UA and AA offer upgrades based on miles but they include a co-pay except for the highest fares. Depending on what status level you reach you can get upgrade instruments on AA or UA or AC (though AC's now come with a copay now destroying most of their value). If your flying pattern happens to fit the fares these things are valid on and you earn high enough status then they can work out well. If you miss out on the status level needed then you'll be much worse off.

In short, if you travel 100k miles a year then you have many options. If you travel less than 50k miles a year then you have very few options. In between, especially if you're booking premium economy, I think BA is the best choice. Even that is kind of fuzzy now that UA and DL have gone to revenue based qualifications.