That is great so long as you have a huge supply, 10K rounds or more, because 5.7 is not a common round. Some other calibers would be easier to get more of.Stopping power is another interesting factor. It seems tempting to pick a weapon chambered for a 5.7x28mm like a PS90 because the only way to "stop" a zombie is to put one in the brain (and their heads are pretty soft from necrosis) and you can carry a lot more 5.7x28 than 5.56x45s.
.223 and/or (5.56)
to name just a few.