My logic on having an additional AR was that they would be different but run the same ammo.
I started with a RRA CAR15 heavy barrel 16". I liked it well enough, but slightly after that I built a STAG M4gery (M4 style upper). I then decided that the next one would be a mid length. I decided to go with a RRA mid length and that would give me Two rifles with matching lowers (ie STAG and STAG and RRA and RRA Uppers and lowers).
This way I have one large ammo stash that feeds both rifles. Both rifles are as lethal as each other. Both had similar shooting feel. Last but not least, most if not all parts are interchangeable.
I'm not really a survivalist "nut" but I do like the idea of being prep'd if the Poo does with the fan.
By the way, I highly recommend the Mid length upper! I love the middy in looks, performance, and efficiency. I picked up the RRA, but would have loved to have known that Bravo Company was going to be offering their mid lengths. The Bravo Company uppers have most of the same quality parts and testing as the LMT do, not to mention the 1/7 twist barrels.
Long and the short of it is this:
Build your own the first time and make SURE you get the parts you want the first time as well. I could have saved myself some cash by only buying what I wanted instead of buying stuff I would be changing out later.
Examples of this would be handguards, grip, Buffer, and stock.