First of all, I hate being the one to say this...I really do, but did you try searching this forum yet? This particular comparison is discussed to great detail many times. I chose JB cases over Justis based soley on this forum alone. And without having to ask. Simply because there is a wealth of information on this topic available on here.
That being said....I never weighed in on my opinion regarding the two. I own both.
Before I decided on a high end case I researched all the major brands, vintage and current. Including Whitten, Murnak, and anything else I could find on here.
Time and time again I returned to John Barton (JB Cases). His personality can be a bit abrasive on this forum at times, but in all honesty it is due to his passion for his own cases and how strongly he stands behind them. This is indeed a great thing.
I own a vintage Jack Justis case as does my brother. These were when the cases were made with individual PVC tubes. The older Jack Justis cases are indeed a beauty to behold. They are not very varied in their appearance, even today, and are generally easily spotted in a crowd. That or a copycat case trying to emulate Justis. To this day his cases look very much like they did twenty years ago. Which is actually a great thing since his look and design is his signature and what most people want in a Justis case. The interiors were terribly lacking. A cheap Porper from the same time period offered a better level of protection. But jack has long since updated his interirors.
John Barton on the other hand has very different styles and looks. He emulates many different style cases, including tributes to great case makers gone by. His construction and style is extremely varied. Probably moreso than any other case maker out there. John is willing to accommodate virtually anything you can dream up. He has done exhaustive work with virtually every single case available today as well as many cases no longer available. If you're looking for a Justis style look, Justis is the way to go. If you want something different, JB is the only way to go.
But there is one defining difference to the two new cases offered by these two case makers. The interiors. With the most sincere and greatest respect, I must say that, after seeing both of these maker's most current offerings, the JB interiors offer a far greater level of protection for you cues. I feel the Barton interiors are simply better. Better padded, better constructed, and a far greater attention given to what will be going inside the case as opposed to just what the case will look like.
There is no way to say these things without someone feeling slighted. And I do not mean any disrespect to Jack Justis whatsoever. If there is one caveat to the differences in these cases I would say that, for collectible and value purposes, Jack's cases will undoubtedly hold their value much better. if only because Jack does not produce anywhere near the quantity that John Barton does. And there is definitely an instant value given to the "Jack Justis" name.
I guess it comes down to practicality versus collectability. If you want a case that will be worth more a few years from now, possibly even as soon as you get it, The Justis would be the way to go. But if you want something that will offer your cues a better level of protection as well as having something very unique (versus getting a trademarked look/style) then JB cases is the way to go.
FWIW, unless you're specifically looking for that "Justis" look, which jack obviously does better than anyone, then you have a far greater variety in Choices dealing with John Barton. I have bought two cases off him in the last few months. You simply can't go wrong.
I know there will be some people to argue the opinions and conclusions I just made. That's okay. You asked for opinions and you got mine. if you search the forum you'll find dozens of others, from both case makers as well.