Whether to use a shotgun or a rifle should depend at least in part on how close the neighbors are and how fond you are of them.
This is one of the most common myths out there. No offense, really. There is no meaningful penetration difference between buckshot and .223 rounds. Both will go through several (at least 7 or more) normal house walls before coming to a stop. Look for yourself:
The Box O' Truth - Ammo Penetration Testing
Stopping power is also an issue
And there really is no meaningful stopping power difference between buckshot and .223 ammunition. There is no clear proof that the "loss" of kinetic energy via over-pentration plays a part in terminal effectiveness (In ballistics science, this is a controversial point). Either way, buckshot stands just as much of a chance of failing to stop inside the bad guy as does .223 ammunition.
The most important thing, actually, is to use the tool that YOU are comfortable with, that you have confidence in, and that you shoot well.
Assuming that I shot equally well between the two, I'd rather have 30 rounds of .223 coming out of an AR-15 with an 12.5" barrel and a suppressor (compared to a shotgun with an 8 round magazine).
There is no meaningful penetration difference between buckshot and .223 rounds. Both will go through several (at least 7 or more) normal house walls before coming to a stop. Look for yourself: The Box O' Truth - Ammo Penetration Testing Yes, a 00 buckshot load will penetrate seven sheets of dry wall. That means it goes through three walls, loosing energy all the way, and lodges in the fourth.
As shown here.
5.56 mil on the other hand penetrated 12 sheets, six walls, and kept on going. I'd call that meaningful, as in at least 100% more penetration with 5.56 mil versus 00 buck. (In the same test 5.56mm also penetrated twelve 3/4 inch pine boards. Doubt I could do that with 00 buck.)
As shown here.
That translates into about twice the chance to kill someone you can't see and don't want to hurt downrange.
For apartment and condo dwellers there is an added factor. My apartment, like many others, is separated from the neighbors by a fire rated wall consisting of two sheets of dry wall and two sheets of 3/4th inch plywood, plus insulation. In my case most of the walls are also armored with about six inches of books. I am reasonably confident; confident enough to risk going to jail or facing a civil lawsuit that would break me if I ever have to shoot and I'm wrong, that this barrier will slow down a 00 load enough to prevent injury to the neighbors. I'm not so sure about a 5.56mm round.
There is also the fact that I can't legally own any rifle with more than a 10 round, internal, non detachable magazine. Where I, and about 36 million other people live, a shotgun is my best choice for home defense. I really have though this through.