I saw both of them in small clubs during the glory years (late 1970s), and conversed with both of them. (And got to know Jaco a bit over the next couple of years.)
To me, Jaco was more of an innovator, with an incredibly original vision and idea of how to get sounds of the instrument, etc., but Jeff was (is) more of a dedicated player.
I saw Jeff improvise a solo on the standard, "Joy Spring", that went on for about five minutes and showed absolute mastery of bop harmony, development, and of, course, ridiculously fluid horn-like chops. He was sitting down with a copy of The Real Book on his lap, open to that page. NO other electric bassist at the time could have approached that.
Jeff has been an obsessive practicer for decades, while Jaco freely admitted that he put in about one year of intense practice (in 1971, if I recall correctly).
As far as the tone, I agree with the person who said that Jaco pretty much showed everyone the way, with his bridge pickup thing. It just lets the instrument "speak" so much easier.
After I was first exposed to Jaco, I had the reaction that people had of Hendrix: "What planet is this coming from, and how can I begin to understand that?" After that evening of Jeff Berlin and "Joy Spring", my reaction was, "I've gotta PRACTICE!" To me, that's the difference between them.