Sound engineers probably love P and J basses because they have a clear idea how the sound envelope feels. They may even already have settings pre-programmed for both. We have a few weirdos in that family (the 70's JB and it's bridge pickup position) but they're in known territory. And very often they will have a P and J bass at the studio available "included in the rent price". If you bring out a weird bass you built yourself, they are going to wonder how it sounds, how good or bad it is, and because they have to mix it later and work on it they won't like it as much as a P or J bass I guess
I started with a J bass and I bought a P bass later. I like both, they have a different timbre.
I usually play the J bass but switch to the P when playing a song where the bassist was using a P bass.
But since I can get my J bass to sound somewhat like a P bass by tuning down the bridge pickup... (I said somewhat... it's not a real P bass sound !!) sometimes I don't even bother and keep the J bass in hand.
If the sound enginner asks I can make two takes, one with each bass, and let him choose the recording he things cuts out the best in the mix (I use he but it can be a she of course). Both are fine instruments, and I will leave the recording/mixing part to the expert I'm far from being and follow his advice. What I will play with the band "live" will be my 70's J Bass but live is another beast anyway. The guitarist will miss notes and play badly, helped with his alcohol level, and playing properly everyone will be very happy of me. Especially the bar owner for me not having vomited on the stage and/or the public.
As a non-pro musician I think listening to experts is sound. I play with musicians that are, like me, amateurs. We learned to play, and enough so we can play in public, but we are not good enough to make a carreer out of it. We all have main jobs, and music is a hobby. So if a sound engineer asks me to make a take with his studio P-bass I'll do it. I'll play the same, hear the song with a different tonal feeling. Doesn't matter if I like it or not. I want the sound engineer to be happy, so what I do has a nice spot in the mix, is easy to work with. When we do recording sessions, I see my job as working for the sound engineer and I listen accordingly.
There's no worse medic than a medic they say
