Not exactly sure where they differ. But the most obvious thing is that Snappy has a completely different way of installing packages and its own package format, too.
HypriotOS on the other hand just uses the standard Debian way of installing and managing software.
Everybody who is interested in test-driving Docker 1.6 in a really easy and fast way and has a Raspberry Pi lying around should have a look at our prepared Docker SD card image.
Currently this is more of an experiment to see if it is possible to get Docker running. The image is based on Raspbian with a custom kernel compiled for working with Docker and OverlayFS. We will publish our buildscript within a couple of days on github. Based on the feedback we get we want to provide updates to the image on a regular base.