I agree that the Midlength is "just technically better" and by that, I mean in theory and on paper. In reality, there isn't any data/evidence to support the claims that the service life of a bolt or any other components is increased with a midlength over a carbine. The theory and the claims are there, but the hard data isn't. Maybe 10 years from now, enough people will have shot Middies to death so that there can be a good base for comparison, but we're just not there yet.
As for shooting "softer" that has always been a bit of a joke to me. I've never fired a properly built 5.56 carbine that had recoil that could be reasonably labeled as "harsh." In my own experience, I've compared my 6920 with factory H-Buffer, my Daniel Defense 14.5" Midlength with H-Buffer, and my BCM 16" Midlength with H-Buffer, and they all shoot soft. Any difference in felt recoil was negligible. One key point in this comparison is that all three of these guns are properly built carbines from quality outfits. If someone were to compare an overgassed DPMS carbine to a Midlength BCM/DD/etc., then the recoil might indeed feel softer. How smooth the bolt cycles can also alter perceptions of recoil, but that's getting into another thread topic altogether.
Properly built carbines run, and they run well. They are a well known and understood specification. It's interesting to note that when Daniel Defense teamed up with Vickers for his signature model, he chose a carbine length gas system. That guy has forgotten more about ARs than most of us will ever know, so his choice is a clue.
I only own 2 Middies right now and no carbines; the 6920 was traded off. That more has to do with the fact that I liked the overall setup of my middies. Might be picking up a PSA carbine build tomorrow though, so my money's actually where my mouth is.