While RancherOS will probably work better with Rancher, the main reasons for us using CoreOS both centered around our using VMWare. CoreOS comes as a VMWare OVA, so installation was simple; also the CoreOS OVA has VMWare Tools installed and configured so that the VMware monitoring gets all the necessary data. Were we using physical servers I would most likely go with RancherOS; if RancherOS came in an OVA with VMWare tools then we’d be using it.