You need to play them all and make your own choice. They are all great.
used market or new?
I have owned them all. Regular JP6, BFR and all the JPX variants. So far the JP12 is the best one for me. Just has the perfect tone IMO. Tight, yet smooth and fluid. Extremely versatile. The JPXI is a close second. The BFR's are gorgeous with the sweet figured tops, but IMO the JPX's play and sound better. Don't look to shabby either.
So hard to say wich one is the "best" ... I played the JP6, JPX and currently a BFR.
I miss my JPX dearly but the BFR is my favorite so far.
But overall I would say you can`t buy a "bad" JP model, because they are all so so so good.
No matter what gig you play.. the JP does it all!
but is the price diff between the jp6 to the jpx/xi/bfr really worth it?
My opinion? YES!
My "Black Sugar" has a beautiful roasted Birdseye-Maple neck. I stripped off the coat and waxed it, feels like a baby`s bottom now and looks gorgeous too!
The finish of the body sparkles like crazy on stage but looks decent and pitch black in normal lighting.