If you can fly 25,000 miles on a single carrier/alliance, that comes with things like priority seating and extra miles, so it would make sense to focus on one.

With your revenue mix, though, it's hard to say where you would come out better. At UA, a flyer under 25,000 miles gets 5 miles per dollar spent, less taxes. At AA, you get 100% flight miles based on distance. A round trip LAX-BOS that was purchased a week out for $550 would bring in over 5,200 miles at AA and 2,500 miles at UA. Buy a $1,050 ticket instead, and AA remains at 5,200 miles while UA gets close at 5,000 miles. You pretty much need to get a spreadsheet out and sort out where you bring in the most miles with your fare mix.

If you reach 25,000 flight miles in a year there would be less competition for upgrades at AA, as that carrier does not automatically put all elites on the upgrade list for flights over 500 miles (UA puts all elites on the list). AA requires upgrades to be supported at the 25K level for flights over 500 miles while UA comps them all.