IMO if you can reach and maintain EXP on AA (and everyone should!), then choose AA as that is measurably better than UA 1K.

If you're mid-tier or lower on either, its more of a toss-up. E+ is offset by what I believe are slightly better redemption opportunities on AA. Mid-tier or lower I'm not sure which program I'd choose but prob lean towards AA. They just seem to have 'cheaper' award ticket availability (sAAver) more so than UA, but I haven't done a scientific survey.