Hey so I actually went through that exact same decision. So "Rancher" means one of at least two things (I'm not an expert but just tried really hard to RTFM) -- RancherOS or Rancher the container platform management system -- for those who may not be familiar.

As another person noted Rancher sits at a level of abstraction ABOVE kubernetes -- you can set up Rancher to work with Docker Swarm, Mesos, Kubernetes, and Rancher's own libraries. Since this was how I understood it, I went with just vanilla Kubernetes first, and figured that once I got really comfortable with that, I could throw the Rancher UI on top of it (the install documentation is actually super duper short, it's just another container) when I got tired of typing things in at the console.

I didn't opt to go with RancherOS because CoreOS was already a step away from what I knew -- most of my machines are either arch, alpine (inside a container) or debian/ubuntu. RancherOS takes the stripped-down OS paradigm even further by dockerizing a bunch of system services -- I wasn't quite ready for that. CoreOS not coming with a package manager is already jarring enough for me (I'm used to it a little bit more now) -- I wanted to take a small step rather than a huge one.

Rancher actually uses Kubernetes, they're slightly different tools.

Process of doing which part? An OpenFaaS backend?