Yes, I agree with your summation of the P vs. J thing. The Jazz is more articulate, the P is more chunky.
The P/J is a strange beast. I have never been totally happy with this setup. The first problem has been to find a good pickup for the bridge. I don't like having a single coil pickup stuck in there all by itself humming constantly. Many companies (including Fender) put out their P/J basses this way. Until I recently "discovered" the DiMarzio DP149 Ultra-Jazz pickup, I didn't know of a good single space humbucker that sounded good and would match the output of a P pickup. (I like the Carvin H50, but it is really meant to be used with a preamp, in passive mode it's not quite loud enough for a stock P pickup).
The other problem is pickup placement. My experience with several P/Js has led me to believe that you need to place the J pickup closer to the bridge than you would if it were a standard J/J set up. The standard J bridge position sounds too nasal when combined with a P, placing it approximately 1" from the bridge gives you better high frequency blend with the chunky P pickup. However, most builders default to the standard placement.
As far as the neck goes, I'd personally opt for a thinner Jazz neck on a PJ, just my preference.
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