JeOS it's a striped down version of a full blown linux bistro. Think Like RH,Centos. Can be used as a VM os or for anything else.
MicroOS it's a cloud OS linux, more like: CoreOS, RancherOS, AtomicOS. I think it's a default choose for Kubernetes environments.
I've been using JeOS for k8s hosts of Rancher environments since RancherOS was deprecated and I'm glad that they finally released a cloud OS like MicroOS. In case of k8s upgrades, rollbacks and upgrades need to me minimised as much as possible. This OS allow to have all of that in a breeze since it's all based on static images.