We recently spoke with a company that is evaluating Rancher and wanted to know how it compares to Spinnaker. Here's how we think the two products compare:
Rancher is not a CD tool. Rancher appears to be a container management system built on top of Docker similar in nature to Heroku. While you might find value using Rancher to manage your Dockerized applications in production, it will not help you get your applications into production.
Rancher requires a LOT of glue code to achieve CD. I downloaded their CI/CD playbook from their website and there are 25 pages of documentation on how to implement CD through a combination of Jenkins, Docker Compose, and Rancher (pages 21 - 46). This is a lot of work just to get CD up and running using Rancher and it is still missing key features that Spinnaker offers natively (multi-cloud support, immuttable infrastructure, clean UI, etc)
Rancher does not support blue/green. Rancher requires you to install an agent on each host (the opposite approach to immutable infrastructure). Blue/green comes for free with Spinnaker out-of-the-box because of how it approaches deployments.