But to me it is really a moot point. Unless you are shooting full auto and don't like to carry spare bolts, a carbine will serve you fine.
+1
As for your question about the midlength making the rifle last longer and more reliable: maybe, maybe not, in that order.
I know a lot of folks are buying into the idea that they might last longer and honestly they may in that respect, but some folks don't like them for reliability reasons because of the gas port placement (specifically less gas dwell time in the op system) because they tend to cycle "softer" will make it more likely to be finicky in "not so ideal" conditions, and it would seem logical the gas port sizing and the gas tube itself would play a large role in how long it would remain reliable.
Honestly just sitting and thinking about how things work, would seem the gas port would erode slower in the middy which is a good thing, but since there is not as much backpressure being blown through the gas tube, would be easier to become clogged with use and not cycle as well if it or the reciever innards are not maintained like it should be.
I suppose the gas port could be drilled larger to allow more pressure to go into the tube (and it may be but I don't know one way or another as fact), but the port placement being so close to the muzzle still won't allow as long of a duration of the pressure curve because it's closer to the end of the barrel no matter what.
I'm not saying a middy is a bad choice all around or anything, because I don't know what your expected use is, but as with anything, there are pro's and con's to everything and you just have to weigh them and decide what's best for you. I won't be mad no matter which one you buy :lol: Get what you like and feel most comfortable with.;-)