Another announcement about containers went largely unnoticed: Ubuntu released Ubuntu Core, a Ubuntu variant optimized for cloud and IoT security. But the documentation and developer adoption seems to be behind CoreOs currently.
I haven't done anything with CoreOs yet, but from my personal feeling I'd prefer Ubuntu over something that is based on Gentoo. I may be completely wrong on this though.
I also not a guru but I've been locking on Ubuntu Core and it seems even more simplistic than Core OS. The great thing on Core OS is the etcd (for exchanging key values across clusters) and fleet (for managing e.g. containers on systemd and etcd). So Core OS has better management tools for things like Docker. Ubuntu Core is missing both parts and it seems better for embedded systems than for cluster management IMHO. And many people are using Ubuntu INSIDE the container. CoreOS is a good minimal layer... no need to have Ubuntu there... also Ubuntu Core does not have tools like apt-get ;)
If you are 100% Containers (Rocket, Docker, etc) the Base OS matters the least. From that you should be concerned about security upgrades to the base OS.
But having another layer of complexity where people run half Docker Half Snowflake will eventually burn you like running MongoDB with MySQL on the same ec2 instance.
I haven't done anything with CoreOs yet, but from my personal feeling I'd prefer Ubuntu over something that is based on Gentoo. I may be completely wrong on this though.