OK, so besides the brand bias flamming - some of the other features of an AR15 you might want to consider are:
1) 16" carbine. Remember, it is only a 5.56mm cartridge. Just becuase the sights are calibrated out to 800 meters, doesn't mean the round is effective that far. Inside of 200 meters, there is little ballistic difference between carbine and rifle. but the carbine is oh so handy. You want to shoot somebody at 600-800 meters, use a 308 (or better). And a scope.
2) Flattop. This will allow you more varied sighting options than the fixed carry handle. And you can always mount a detachable carry handle if you wish.
3) M4 handguards. For improved heat disipation, but also because they feel better in my hand.
4) If its postban, go with a bare muzzle. Muzzle brakes are just plain LOUD. 5.56 recoil is negligible anyways.
5) Again, if its a postban, go with an A2 stock. The fixed telestock is, in my opinion, stupid and for posers. You'll get a better cheek weld with an A2 stock.