Normally I'd start by recommending that you consider which route structure fits your travel patterns best, but as previously posted that seems to be about a wash.

Two factors you might want to consider, one in favor of each and both depending on what you think the future holds:

1. If you expect to fly on economy fares, elite status with UA will get you E+ seating with enough additional leg room to notice. Non-elites have to pay for that: on a per-flight basis, or $349 per year for themselves and a companion.

2. If you expect your travel to fluctuate a great deal, AA has a "soft landing" approach where your status drops at most one level per year. That can carry you over low spots without returning to a dreaded status-less existence. UA will drop you from super-top-tier to nothing if you don't fly.