Marion's is significantly more consistant than Cassano's, I'd say you get roughly the same quality pizza every time you go to marians, at at all the locations I've been to (roughly 5 or 6 of them), whilce Cassano's has always struggled with that. A top shelf Cassano's pizza is better in my opinion, but you only get that top shelf Cassano's pizza about half the time, the other half of the time, I'd rank Marion's above Cassano's (though the cassano's pizza on the off days is still pretty good, it mostly has to do with the amount of salt underneath the crust, which plays a huge role since the crust is so thin)
They are building a Marion's is Mason for anyone in that part of town.
For (south of) Dayton area pizza, I'd go:
Monster Pizza (aka Doubleday's in Centerville)
Jets (which has a much larger footprint than I realized when I first had in columbus a few years ago, I know its in the Cincy area now)
Dewey's (though I consider Dewey's more of a cincy thing since it started there, even though there is one in Dayton now)
I could flip those 3 around pretty much anyway depending on the day, Marion's is great for dinning in, cassano's if I want delivery, Dewey's is better for its uniqueness and their salads and beer are great, so it makes for more of a meal than just going out for a pizza.