LeftyBassist.com

As I now have one of each in my possession, it's time to do a comparative!

First! The specs!
Both basses are very similar featuring an Alder body with Maple fretboard, the JB2 is a gloss neck and the JB has satin - subtle influence on tone.

JB2
Sonic Blue Frost over Alder, Light Tint Gloss Maple neck and fretboard, 7.25" radius with a 1.5" nut. Currently strung with EB Super Slinky bass (45-100)

JB
Bel Air Green Frost over Alder, Satin finish on Maple neck and fretboard, 9.5" radius with a 1.5" nut. Currently also strung with EB Super Slinky bass.

Playing!
From a play-ability perspective, I find the JB2 to actually be easier of the two to play. It has a balanced weight from headstock to heel that just feels right, I find that's true of all L-series bodies. I think there was good reason Leo Fender was so proud of this design, it just hangs like it belongs on you.
The JB feels like a classic jazz bass in all regards, the body weight gives the neck that nice upwards tilt that helps with moving my hand around the neck. The G&L high mass bridge is probably the biggest difference maker over the traditional Fender Jazz bass, there is a lot of clarity and sustain in the notes especially when you get into chords and harmonics.

There is a very tangible difference between the two neck profiles, the 7.25" actually feels a little too small to my hands these days. 9.5" has a great rounded back with lots of meat to hang your thumb on, that let's you relax and move a little quicker once you get used to it. I find with the 7.25" that I tend towards clawing in with my hand instead of spreading out comfortably.

Tone
Fundamentally, there shouldn't be any major tone differences and for the most part they are very close to each other. Both basses feature G&L's own An5 recipe Jazz pickup designed by Paul Gagon with the same body wood materials, neck, and bridge.
But there is a definite variation between the two basses, the JB I would describe as being far more authoritative and deeper sounding across all the strings, where the JB2 is brighter and has a lot of upper register chime to each strings attack.
Body mass is the only major difference I can think of that would separate the two, the JB is easily heavier by a half pound. Also after measuring a couple times to be sure, the JB2's bridge pickup is a 1/4" closer to the bridge (almost at 70's Fender jazz position) where the JB is definitely in the 60's Jazz sweet spots.

Nice review bigevilrobot. G&L has something here and I agree the mass bridge chimes out the notes quite well. My JB5 had a really good B string that had that piano ring to it. I have not had many basses that can do that especially in a 34" scale. Great work at the G&L Factory.

That right there would conflagrate into a weekend-long flame war on TB. But here we'll just say, "Hmmmm, interesting."

Edited to add: thank you for the review. I especially appreciated your impression of each neck.

Oh man would it ever! I basically won't post on Talkbass all that much now because of such strongly held opinions from both sides of that argument.
When I say subtle, I mean mouse-ears hearing subtle. I'd actually say that it more influences the feel than the tone, but even that I suspect is mostly subconscious.

Have you ever played an MJ? I am curious how their active electronics match up against their passive electronics.

thanks for the review, very informative.
would it be possible to ask you to post some pics to this thread for lazy 'effers like myself who won't search other threads (lol)? would be very interested in seeing if pics can pick up the differences in neck radii.

I found my JB2 a bit overly bright until I swapped the pickups for a set of 2008 UDA standards. Made a big improvement. The bass has more growl, is sweeter and not as shrill. I found the passive electronics work really well with these pickups too. Tons of range on the tone pot. I can only imagine how good a set of boutique pups would sound.