With the three that you listed, I personally have a hard time with the $700 dollar difference in the Colt over the other two, other than the fact that it has some listed upgrades in the trigger and butt stock that the others don't...I'm just not sure that the "Colt" name is justified for the additional cost if the cost of the upgrades were applied to the others...
I've been a DPMS fan for a number of years and my 3-gun competition action shooting AR has several upgrades that enhance the accuracy and durability over the stock production version and I've had, and rejected, several other brands over the years for specific reasons relative to the individual make/model..I still have the DPMS...
If you are considering, as you should, a potential resale value, the Colt name is hard to argue with as there are 'purists' out there that will accept nothing less...Whichever one you choose, I think you will be happy....