To the OP. I have a JRC 9mm and theuron 45. Like another post said the cost is not much between 9 and 40 but 45 can get a bit pricy sometimes. They are a heck of a lot of fun and the fact that they take glock mags is a plus with the 10 round goodness

. Lots of Pros to shooting a pistol carbine. Less parts, as most are blow back operated they are more simple design so that can be a plus. The ammo is smaller so you can pack more in one can than lets say even 5.56 or .308.
Much less recoil. Lighter weight, ect. plus the free floating 18.5 inch barrels really highlight the ballistics of the smaller rounds. Very accurate up to 100 yards no problem. IMHO theuron is best design as far as being the most user friendly.