Second ditto on Troy, and a big fat ditto on KevinB.
Plus, you get a longer handguard area. I know that sounds elementary, but you have an extra 2" to put your hand forward without burning yourself, you get an extra 2" to mount accessories (if that's your thing), you can put a bipod 2" farther forward if you use one...
But MOST importantly, you can use a regular bayonet without addons or doodads, just in case you ever need one to rob a bank or pillage a village
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These days, most everyone (except KAC
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) makes stuff for the mid-length. If you want free float, railed forends, there's everything from sub-$100 YHM or Oly, to the $300+ Daniel Defense, all the way up to the ARMS SIR system.
If you just want regular handguards, well there's the standard carbine handguards from several places like RRA or Armalite, or if you want M4 handguards, there's Cavalry Arms.
If you can't tell, I'm a little biased. The middie is my favorite by far.