threads like this are gonna rack up post numbers....QUICK
main difference: BJJ is guaranteed to work, mat doesn't lie...
JJJ on the other hand, it depends HIGHLY on who's teaching, some just teach katas and pressure points and never roll live since their art is too deadly, some crapple (spar but their ability in sparring is NOWHERE near that of BJJ) and a few places that are actually legit, but good luck finding one. I would say something that's common in most JJJ schools is that they stress memorizing a lot of moves as opposed to a few that you're going to keep using again and again (in BJJ)
Three cases of JJJ guys I've sparred with
One of them trained for 2 years, i grappled with him when i totally sucked (less than 4 months), threw him, mounted him, armbarred him under 10 seconds, i actually wasted two seconds wondering if he really stuck out his Armbar under mount since he supposedly did a grappling art for two years....
The other guy, started JJJ a year before I did any grappling. last time i saw him grapple, he got mauled in the takedowns and guard passing because he was virtually clueless with the No-Gi grips, but he has an annoyingly strong turtle and he's almost impossible to submit.
o and I did get subbed during my first two months of training by this JJJ guy who was hell belt on pulling the guillotine choke
I wouldn't want to say that BJJ is "better" but I can definitely say that your chances of getting good training are MUCH higher with BJJ