Other than being AR15s, these are very different firearms.
Colt will use forged aluminum parts in a conventional design. While the attention they pay to cosmetic elements can be pretty low, the functional quality, lifespan, and durability of their individual parts is excellent. The colt will have a conventional chrome lined, button rifled barrel with a carbine length gas system. The production process and testing of the major parts is well-documented.
The UU will use billet aluminum parts with a really great rail attachment mechanism. It will have a button-rifled stainless barrel and a midlength gas system.
Either barrel will last well. A chrome lined barrel will handle abuse like heat or poor maintenance better, but the LT barrel will almost certainly shoot better and is pretty hard to mess up.
The Colt will retain a greater percentage of its value, if that is important to you as Colt parts are highly sought after in the resale market and the future supply of Colt rifles to the consumer market is uncertain, if that matters to you.
I think the UU is a better value as a shooter for almost all purposes, the 6920 would be the choice for a duty gun or a collector piece.