This is not correct. Since these come from different inventories, it depends upon whether both are available. If both inventories are available the EXP will, in general, have priority. As it should be.
Cheers.
Really? I would think if both were available AA would still prioritize based on the instrument being used. If not, why wouldn't EXPs always use miles to upgrade, then get confirmed and once X/R opens cancel the award (which they can do now)? Even if they lost the co-pay, so what, small price to jump the queue.
I also remember reading something here (not 100% confirmed) that even within C/A they will prioritize mileage upgrades over SWU upgrades (regardless of status), so I would think that translates to this as well, but I could be wrong.
As far as "should be" I think it should be prioritized by type of instrument used. If there is one "A" seat left, and there's an EXP wanting to use a SWU, a PLT wanting to use miles and a GLD wanting to pay an A fare, I think it should go to the GLD (assuming a valid A fare exists for the route), even though all three are wanting the exact same inventory. Priority should go first to those giving up the greatest consideration (or that which AA most wants to get on/off its balance sheet) first.