I'll have to agree with MADWABBIT. When it comes down to it and the initial chaos has subsided... it's going to be whatever your can find ammo for. The weapons that aren't "main stream" are going to end up being the weapons of choice considering all of the common ammo will be gone in very short time... if not hoarded by other shooters, they will be shot up.
Anyone breaking into stores to pull ammo is going to grab as much as they can of what they need... not the uncommon stuff. If you have a weapon capable of using the uncommon ammo types, you'll have an increased supply as that stuff will be left behind. No one wants to carry the extra weight of ammo they won't use (yet).