I never said the 40,s werent good, just that I like the better top end response of the 41 diaphragm. I also have several 40,s with 41 phragms and to me that sounds really good. I have stated this in other posts as well. The 41 phragm in a 40 driver just sounds really good.
I have heard the 2450J and 2451J and it is smoother than a 2446. But this particular neodymium is not my favorite, The TAD 4002 is. neodymium with Beryllium diaphragm! The beryllium diaphragm inside the TAD driver has no ribs, its a smooth surface dome.
Crossover has much to do with everything, as Robh already said, and your expert is telling you.
My guess is that for the definitive answer, why dont you call JBL, explain to them which horn/driver combination your using and ask what they recommend!