The new megalithic (mts) is an absolute work of art. It is one solid piece of billet aluminum. Only comes in rifle length which I'm not 100% certain the actual handguard length. Probably about 12 to 14 inches. Also is about 650.00 dollars
While I am a fan of aero and own there parts there cop platform is not a technically a monolithic upper but I don't think it matters. It bolted together is beefed up and it is very strong. The picantiny rail is monolithic but handguard pieces together. Probably not a big deal. Aero is also quite a bit cheaper and the ability to choose from different lengths and handguard sizes is nice.
I am on the fence between mega and aero for the same uppers you are considering. Both make solid products. Mega will likely have slightly better finish and will look best with a billet lower especially there gtr- 3h
Best of luck
I'd seriously consider aero. Good solid products and price. Personally I like the cop aero look and not many out there so it will likely be unique. Also it may fit your short barrel best