I play U.S. Fender and G&L basses, and I don't think it's so clear cut in G&L's favor. The revisions to the American Standards for 2008 bring them much closer to G&L for build and finish quality, and the newer higher mass bridges are more like G&L's heavy duty saddle lock bridge. My 2008 Jazz V is every bit as nice as my 2007 SB-2. That said, G&L's finishes are still better than Fender's, and Fender doesn't even come close to matching G&L's options unless you go Custom Shop.
In my view, G&L's biggest objective advantage is the Plek fret levelling. I have two G&L's with Plek treated #8 necks (1.5" nut, 12" radius like the JB-2), and the playability is unbelievable. My 2003 Fender Jazz plays almost as easily as my G&L's, but not quite, and it's a meaningful difference.
Beyond that, I think it's personal preference as between JB-2 and Fender Jazz. The G&L neck has a slightly meatier rear profile, which many players prefer. The JB-2 sounds like a Jazz, but with a slight modern twist, probably from the 21 fret neck and six bolt neck joint, but maybe the pickups too. As others noted, the JB-2 doesn't have a Jazz body, so it will feel different. Some prefer the difference; some don't.
If I were getting my first J, I'd probably custom order a new G&L JB so I could pick one of G&L's amazing finishes, get a matching painted headstock, and have that killer playability on a traditional J.