At this point, you have to pick Mercer. Joshua still has some improving to do. Ask again in a few years.
The sensible pick would be Mercer by UD or late stoppage, but even prime Mercer had hot and cold spells.
We have yet to see a prime AJ, so with both at 13 or so fights under their belts each, I would go with tough sonofa***** Mercer.