Colt did several things to keep anyone from converting their rifles to full auto:
Larger than standard trigger and hammer pins.
There is a crazy screw system instead of pushpin on the front end.
A big steel block where the autosear would have gone.
Due to this it makes it difficult to interchange the upper halves with those of other manufacturers.
You have to use only hammer and trigger parts made for the large pin Colt.
Due to the steel block, you can't use some aftermarket match triggers.
So, I advise, of the two you are interested in, Armalite.
None of the AR15 mfgs are "milspec". Some are better. Armalite, for example, has a stronger, heavier charging handle, for example.
Also, I will say that Bushmaster, DPMS, and Rock River are equal in quality to the Armalite. New-old company, J&T now has their Double Star line out, and these are excellent. I have a rifle built with a J&T kit (the upper came complete, ready to shoot).
Good Cop: A Cavalry Arms lower with a stainless barreled upper can make an excellent "trunk gun". They are tough as nails and light weight. Cavalry Arms is presently retooling, and has just started producing a new and improved lower receiver. I have a new one coming soon, will post a picture.