They have slightly different internals, main difference is the BWA has the better camshaft which causes less wear on the HPFP cam follower.
AXX actually has forged pistons
....I'm showing my ignorance here when I ask if forged pistons are better or not than whatever pistons the BWA has..... ?
I'm no expert but I think they are better. The weak spot in all these k03 engines are piston rings and Rods, the BWA's pistons are up for at least 400bhp anyway. Maybe VW realised they were over engineering it and decided to save a few £'s