Well, technically speaking a CAR-15 had a 10.5" barrel with a 5.5" flash hider on the end. The M4 uses a step cut 14.5" barrel.
In terms of modern civilian AR's, the CAR is probably going to be a 16" heavy barrel, and the M4 is going to be either a 14.5" step cut with some sort of muzzle brake, or a 16" step cut. The only real differences you will see are velocity (the longer the barrel, the higher the muzzle velocity), volume (shorter barrels, and muzzle brakes are louder) and looks.
In terms of quality Bushmaster is as good as it gets. 4150 mil-spec steel, chrome lined. That being said I doubt you'll notice any huge difference with the J&T (and I believe some of their barrels are chrome lined as well).