Why midlength? Softer shooting with the expense of a little bit of weight moved forward with a little bit longer sight radius.
I would recommend searches here, ar15.com, google, etc. You will get much better answers out there than I can give. Let's just say that I did a lot of reading and that is what I decided to go with for my first AR. I have nothing against carbine length, I just had to make a decision.
Nice build. Would like to know what you think of the Magpul sights. Not sure I like the idea of plastic sites.
I will let you know. I realize many people feel that way and I respect the arguments either way. Lots of individual decisions with AR's -- that's part of the beauty of them.