Keys trip: upper vs middle

[makade]

Anybody got any insight on the middle keys vs upper? I am trying to plan a trip this summer and narrowing it down to a couple of places. One is in Key largo, and then there are a couple of options in Marathon and Key Colony Beach. I'm looking at renting a house for a week. I feel since we'll be there for a while its worth getting further down towards Marathon? Also I have young kids to keep happy, so it's not just a fishing trip. I would like to get on some super shallow reefs for snorkeling with the family. Any advice will be much appreciated.