I chose the AR 5.56 because of stopping power, ammo avalability, range, reliability and general versitility. The AK would ba a good choice also for a lot of the same reasons, it also has the bonus of "stone age maintenance" I have seen an Afgani polish his action with water and sand.
I think the range and power long gun woud be more usefull in a breakdown of authority situation and concealabilaty less iimportant.
In a Katrina situation, the " authoritys" would try do disarm the populace even before re-establihing order. In this case a couple of SKSs ($250 a copy and of course with no paper trail) and a few thousand rounds of ammo for it buried someware close might also be a good choice, because the best gun is the one you have when you need it.