In my mind, the classic example of "urban" is Beslan Middle School #1 [see my post at #59 above].
Then I'd definately go with an ar type platform (or maybe an accurized ak-74). If you have to worry about a civillian anywhere near your target, you don't want to use anything other than a slug out of a shotgun, no matter how accurate they might be. Too many things to bounce around if they miss. Plus with a good red dot type sight, you can have the ability to be accurate 100 yards or more away. You don't really get that with a shotgun.
FWIW, flechettes (& tracer & AP rounds) = illegal to posess in Calif.