Your type of travel, where it's a mixture of economy and first, just about requires you to get a spreadsheet out and look at what you spent the last year, and the distances involved, to see what makes sense.

At the 75K level at UA, you would earn 9 miles per dollar on the fare BEFORE taxes. Look as what you've spent the last year, deduct 5-10% for taxes, and you should come pretty close the the miles you'd bring in there with the 9X multiplier.

At the 50-99K level at AA, you'll bring in 100% flight miles and 100% bonus miles based on distance. On paid first class fares, you bring in an additional 50% bonus on base miles. AA also has some promos out there right now for extra miles on a variety of fare classes, but for purposes of looking forward, where they might not be in play, probably best not to add those in.

One big difference between UA and AA is that all elites get on the domestic upgrade list at UA (which is very different than saying they all get upgraded). At AA, all elites get on the free upgrade list for flights under 500 miles, and those at the 50K level accrue upgrades (2,000 miles worth for every 10,000 flown) or can purchase upgrades, for flights over 500 miles.

Hopefully you can come up with some numbers with this and decide.