I personally like the looks of a midlength better. I think the extra few inches of barrel sticking out of carbine handguards looks silly, and the bayonet (if you like them) doesn't look right on a 16 inch carbine.
I am also given to understand that the midlength is a bit more reliable because of the extra dwell time of the gas system. Some carbines are apparently pretty hard on components because they operate with higher pressure. I am no expert on this, but no doubt someone who knows better will chime in.
For normal range use, I doubt that carbine vs. midlength makes any real difference as far as reliability goes. Another thing to think about is that carbine length is more common so aftermarket rails are slightly more difficult to get for a midlength since not everybody makes one.
My personal preference is for a midlength, and if I had the option of carbine or midlength with the same brand of rifle I would go for the midlength. But this is just my opinion and preference.