are all the same. One may be prettier than another but to be mil spec means the gun is built to military specs. That is why so many guys are building their own from parts. If you look in catalogs of AR parts you will see dozens of makers of parts and they all will pretty much drop in the gun. If I am not mistaken there are only 4 forges in the country that make all the uppers and lowers and they are marked by a code on the right hand side of the upper and lower.
It is a bit of a stretch to say they are all exactly the same but for all practical purposes they are. If you pay more than $1000 for an AR-15 and it is not a special full custom gun, you are getting screwed. There are a lot of $3000 and $4000 guns out there and most are worth it as every part of the gun is a custom part and they have special features most ARs don't have but if it is a standard AR right off the shelf, don't pay more than $1000.