There's lots of posts on this comparison subject elsewhere;
I have a 9mm JR, and my friend has an equivalent Thureon. I've handled, fired and had to clean both and each has advantages and disadvantages.
As far as I am concerned, the initial JR issues are largely gone. I had the early model, that didn't have any problems. But the awesome customer support at JR sent me all the replacement parts anyways; I still have no problems with it. It loves Winchester white box 115gn 9mm.......and is frigging accurate.
This to me is where the JR excels, and the Thureon is unproven. If a person experiences
any issues with their JR, they simply call the 1-800 number for Just Right Firearms and speak directly to the production and service staff. In my books, they are #1 for customer service, and treat all of their customers like gold. They know their gun because they designed and built it, and can resolve any issue, most of the time over the phone.
I prefer the looks of the JR, and most AR accessories fit perfectly. Mine uses Glock mags seamlessly - from my G17, back and fourth; too easy. I can buy mags anywhere, and they are cheap by comparison. When you hold a JR, it feels "beefier"; it fits better for me personally.
I find the Thureon looks really pencil thin and fragile, but that is only a perception on my part. It is well made from what I've seen.
The Thureon that my friend has uses the modified UZI mags previously mentioned. I don't like 'em cause they are proprietary, expensive and hard to get. (This will change, of course, when they introduce the new mag capabilities). But the Thureon has advantages when it comes to cleaning, and the ease you can take it down. It also takes accessories very well, and has comparable accuracy to the JR.
Personally, I think both are great guns; and if you do anything, get one of each and have fun. But I do enjoy the relationship between Glock and JR - makes for a great day at the range.