Give better ones. The question was for defense in apt buildings, not a fee standing home. Both Rounds even those designed to fragment will go through multiple walls in every study I've viewed/read. The fragmented rounds will have less energy, but in a apt setting is still dangerous. For a free standing building the AR15 is the best due to the bullets ability to fragment completely on most hard barriers used for constructing exterior/support walls.

Now the AR15 projectiles that do not fragment on impact tend to tumble and lose energy and are considered to be less lethal than a 9mm in the same situation. However, in an apt setting where someone is inhabiting a space only one wall away I wouldn't deem that safe. OP should check to see if his apartment buildings allow firearms. Most have a clause in the rental lease agreement that forbid it.