Again, Cecil is spot on. This comes with a caveat though, in my very humble opinion: Cecil—and I've seen him teach, myself--is a BJJ God. For him, the perceived advantages are minimal, because his skill set all but equalizes the platforms. I believe him when he says that the J's advantages are overblown.
That said, for a guy like me, who is a BJJ dilettante/retard, there is something to the fact that the J-frame is harder to disarm—exactly because I have not yet developed the skills that Cecil mentions. To wit: in the grounded 1-on-1 evo starting with one hand on the opponent's gun, from the bottom position, I had a pretty easy time stripping/freeing up the j and giving my buddy the business with all 5. I mean, that happened pretty quick (thanks to an *extremely* rudimentary grasp of playing a spider guard on my part), at which point a robust drunken scrabble for the remaining loaded sim gun commenced, since the J had done it's job and ceded the floor. When the roles were reversed, the G17 T FX gun barfed relatively quickly (because my hand was on it, go figure) leaving a nice scramble for the j-frame, since it was the only working gun in the fight pretty rapidly. Circling back to “BJJ being a thing” for a sec; this match, between myself and a young Idaho fellow named Matt, was a pretty good referendum on BJJ effectiveness. Both Matt and I fit into that class of people new to BJJ because of ECQC experience—whether ours, or reading about someone else's and wising up—and as a result, we had a number of people who fit into that “first year BJJ” category, with many of us sitting at roughly the 6 month mark. My recollection is that Matt and I have similar time rolling (months, not years) and the Boyd belts were even: I probably outweighed Matt by 40lbs, and he was probably younger by 20 years. The net effect? The guy on bottom ate the other guy. Even a tiny bit of Jits is huge here: when both hands of both players are occupied, the guy on top has weight on his legs, and the guy on bottom gets to use his legs, with even minimal skills. Hence, me stripping the J in more or less seconds and giving him 5 when I was grounded, and him then getting my j from me and putting burn marks on me with my own wheelie when he started on bottom.
Since neither of us was Cecil, and probably won't be (well, maybe Matt will; he's got the decades and smarts to spare), one takeaway from the unwashed fight was that the J was the gun reliably delivering the hurt in both evos. The other is that BJJ is a thing. Jits skill strikes me sort of like money: a lot more is clearly better; any that's available for use beats the shit out of having none; there are problems that even money and a killer ground game can't solve. JMO.
That's my take on it all, from the bottom of the jiu-jitsu ranking hierarchy, with the addendum that #1) I'm still processing and #2) I'm no Cecil on the mat. I hope that makes sense.