I was primarily considering a Daniel Defense DD4 RIS III (16" midlength) and a 14.5" DD M4A1. I spoke with a guy who's worked at Daniel Defense for 17 years and asked him about midlength vs carbine. He said that several tests have been conducted in the past several years and that, while the results are not public, the midlength outperforms the carbine length in every way in terms of reliability and durability. While my carbine gas LE6920 has been extremely reliable, I figure I'll give midlength a go this time, since Daniel Defense basically wrote the specs on how to properly gas a midlength barrel with their URGI contract.
So now it's between the RIS III Mlok, or a DD M4v9 with a freefloat quad rail. I've been partial to quadrails my whole life (my LE6920 still has a KAC RAS M4 and my M16A4 surplus upper will has the KAC RAS M5), I like how bombproof they are and in military testing they still outperform even the toughest Mlok rails tested in terms of deflection. But I realize the future is now and Mlok is the standard. The v9 is also about $1600 while the DD4 RIS III would end up about $2300. That's enough difference for a couple of cases of ammo, or some ammo and a good optic.
A few months ago I was out shooting my LE6920 at the range, with the KAC quad rail and all. Someone came up to me and said how it was a cool "retro" AR, and I was confused at first. When I got this gun and set it up, it was cutting edge. Now it's considered "early GWoT retro". I'm only 30 years old and I feel old.