I am not sure whether you consider a weapon as defensive or offensive really matters. That is simply the application of the weapon.
If you are questioning the potential stigma that might result from using an assault type rifle, sure you may have some problems, but those problems are really going to be a non-issue compared to the legal aspects of why you killed someone. The debate will come down to whether or not you were justified in using lethal force. This assumes, of course, that you didn't have any errant shots that ended up striking a non-combatant somewhere.
If you are concerned with home defense, why are you not considering a nice shotgun? A shotgun will deliver much more effective rounds than either rifle and you have your choice of ammo ranging from tiny bird shot, to buck shot, to slugs. You can employ a shotgun from contact distance out to 50 yards quite easily, 100 yards with slugs - actual distances varying with the barrel length, choke (if any), and load used.
By the way, an elderly gentleman in Garland, Texas used his AR-15 to stop a truck thief. The old guy had been burgled several times, heard the commotion under his car port, and found a man hot wiring his truck. He issued orders to stop and the burglar attempted to drive over the man who simply side stepped as his truck went by. In the process, he pumped 7 rounds of .223 into the cab of the truck. The thief and truck were found a couple of blocks away, the thief was dead.
Nobody had any problem with the events that unfolded. As per Texas law, the truck owner was within his rights to use lethal force to stop a crime at night against him or his property given that he did not appear to have any other reasonable options for stopping the thief. Of course, once the thief attempted to drive over the guy, he had a right to defend himself. I think I was about the only one surprised by the use of an AR-15. I expected the old guy would have used some older style military gun, maybe a hunting rifle or shotgun. Nope. He liked his AR-15 and used it well as well as effectively. No charges were filed against the truck/gun owner.