I don't think there is anything like one of the two cuts through the mix better then the other. They both have different tone palettes and it's up to how you voice that sound in the room or in the mix that will decide wether you'll be heard or felt or not. And of course the sound your fellow muscians occypy in that space will be a deciding factor. The P-bass pu'p is generally a bit (25%) louder than the J-pu's so that has to be balanced as well as the fact that the J-bass will sound very different depending on what pu'p blend you're using while the P-bass with it's simpler layout appeal to many for that reason. There's no right or wrong here IMHO, both are great in what they'll do and the circumstances will decide what's the best choice of the day. You definitely need to own one of both