if you're buying full fare, use United as the MP rates on those are pretty good.

In general, its best to use your native carrier, especially if you're getting status (Even minor status). And w/United, at least if you have one or more of the credit cards, you can obtain free bags, and optionally the club.

A3 has begun to tighten some of the abuse of its program, and I'd fully expect that kind of loophole to be less exploitable in the long run.