I prefer the Precision tone to the Jazz tone. BUT I LOVE the sound of an ash-body-maple-neck Jazz low-B string. The ones I've had have a great barky growly tone down low that just rumbles. No mud, just authority (IMHO). Which makes a better platform for those 5 low notes on the B string, P or J?
Same scale, so it's basically which woods and pickups you like. Kind of a personal preference thing, but for great lows, P beats J in my humble opinion.
Which is better..? Simple answer, a Music Man or G&L 5-string. Now that's what I call a stellar B-string. But if it has to be Jazz or Precision, I'd say the ash/maple Jazz.