You've clearly thought through this matter pretty well. You might want to say what your home town airport and home country in Europe are, as well as where you'll be going most to accumulate your annual 75K (if it is not mainly your home town and home country) since that information could add to the useful feedback you'll hopefully get from other replies.

I can see the reasons why UA makes sense to you, especially in terms of your monthly trips to your home town. It could be that you'd be best advised to go with UA for now and then switch to AA when you move to Miami.

Having said that, I'll provide a few reasons for sticking with AA, even though I realize they might not outweigh the reasons for switching to UA. I base these on my experience as 20 years of 1K at UA, which I brought to an end this year out of frustration with UA, and my nearly a decade of mostly being an Executive Platinum at AA, which I'm concentrating most of my flying on (well, AA and its partner airlines) from now on.

1. Most generally, UA is in a downward spiral that shows no signs of ending under its current management. This is reflected in its deteriorating FF program (with a big devaluation coming up very shortly), poor handling of irregular operations due to mechanical or weather problems, glitchy reservation and other IT systems, and poor customer service that starts at the top with management policies and attitudes. It seems that every few months UA comes out with some change that makes life as a passenger and frequent flyer even tougher. I know that with the AA-US merger there is a good chance that the combined AA will deteriorate also, but it would have to fall a long way to catch up with UA (even as UA continues to deteriorate as well).
2. More specifically, I find international upgrades and complimentary domestic upgrades on AA easier to get than on UA. My experience is with systemwide upgrade certificates rather than using miles plus cash, but I would think that the comparative availability would remain the same either way.
2. AA is introducing far better international business class seats, with all-aisle access, and I believe that all such seats from JFK are now all-aisle access. The configuration is 1-2-1, as opposed to being as bad as 2-4-2 on UA's 777s and downstairs 747s.
3. One upside of the AA-US merger will be more route options, though I can't say whether that will specifically benefit you and realize it won't do any good for travel to your home town.
4. Some UA flights might involve going through EWR, which is a lousy airport.
5. Once you do move to MIA, AA will probably make much more sense.
6. With UA you have to buy a more expensive "W" economy fare to even be able to upgrade an international flight (with, again, such upgrades being tough to find), whereas with AA just about any economy fare can be upgraded.
7. I would think that flying internationally out of NYC you'd have more AA flight options than UA, which would mean at the very least having extra legroom in economy as opposed to the LH economy sardine can.

Again, not sure that these outweigh your reasons for switching, but wanted to make you aware of them. I don't have the links handy, but if you search through the UA, AA and Information Desk forums you'll find other threads that discuss this comparison as well.