Hi Bevo. You've had lots of good answers so far... obviously the Precision split coil versus the two single coils on the Jazz is one. This makes the basses sound quite different. In a nutshell tonally the P is about the "punch" and the J is about "growl". Another difference is the offset body on the J which balances a bit better for some folks.Bevo said:
As far as the neck goes there's quite a bit of variation between years. Typically the Jazz has tighter string spacing at the nut and a fretboard that is narrower (across) but the neck is deeper (through). The neck on my '62 P is almost like a ruler... wide but not very deep compared to my '78 Jazz.