I have two with J width necks and one with a P width neck.. all P shaped bodies and pickups.. all made for me by Mark.
The J necks are very close to a current US spec neck to my hands.. (I don’t own a current US spec J but have tried a few).. the relic process makes them feel very worn in and smooth.. the rolled edges really feel nice.. also the fretboard radius is very comfortable on mine, I don’t know what he uses, it’s more curved than my Stingray but not too much..
Just to throw a curve ball into your dilemma.. I favour the J necks and they are my main two basses BUT.. the P necked one sounds thicker and meatier.. they’re all Rosewood boards and they’re all P bodies with Mark’s standard hardware and pickups so the only difference is the neck mass.. so for a gig I’d favour the J necks.. but recording.. it’s the P neck..
Mark is a fantastic luthier to deal with.. super helpful and reliable.. I can’t recommend him more highly..