Carbine length gas ports are located closer to the chamber and gases are still burning at a higher rate and pressure at the carbine port which sends hotter gases back to the bolt carrier and receiver, and the gas pulse is sharper and more violent due to the pressures. This is harder on the bolt, especially during full auto firing. M4 (carbine) bolt life is less than M16 (rifle length) actions.
The midlength gas, recommended by Mark Westrom of Armalite, is further down the barrel to the point where the pulse is smoother and lower pressured, plus without the extreme heat the carbine gas system presents.
That being said, in civilian, LE, and many military uses, both systems are very reliable for thousands of rounds. You can tune the carbine gas system with buffers and springs to reduce stress somewhat on your components.
Midlength forends are more comfortable to many because they offer more real estate for your hand positions. If you have rails you have more room on the midlengths. Sight radius is longer on midlengths if you use a front sight.
I've never had a problem with a carbine gassed rifle. So I don't know when the bolt will give me problems on mine, if they ever will.