I think overall you get an overall better instrument with G&L simply because you don't pay the mojo/label tax. I'm a Fender fan, but I once owned an L2K (one of 2 basses I regret selling) and can only comment on it. The L2K is a masterpiece, and probably Leo's best (well, maybe second best) creation in terms of technical achievement (I still say the split P pickup, at the time it was created, was simply a stroke of pure genius).
The L2K has it's own voice, but is also a Swiss Army knife of bass tone. Does it sound "exactly" like a P, jazz, or Ray? No, but close enough that the audience really won't know the difference. The construction quality is as good as Fender, if not better.
G&L also has more finishes and neck options. I have played an SB-2 once, and it was excellent. The JB and LB models get a lot of love here on TB, but I've never seen one. And that's probably what this boils down to- just about every guitar store carries Fender, not so many carry G&L.
All that said, the Fender Precision and Jazz basses are still the industry standard, and with good reason. They are reliable, versatile (not L2K versatile, but versatile enough), and just get the job done right. In a world where history and certain aesthetics don't matter and both companies had equal production facilities and marketing, G&L would probably drive Fender out of business. But that's not our reality.