If I had to keep one, it would be the P. It makes me the most money for session work. Those basses are both awesome, and I hate that you have to sell one.
That being said, I have both. My P's are setup with flats and I have steels on my J. As a session player, that versatility is really awesome to have, but my go to is still the Pbass.
It sounds like you may be in the pick up and play mode right now, and in that case it sounds like the J is your pick.
I think you will regret selling either one - just my opinion.