I worked on Mantl for a while last year. To understand the difference you can see Mantl just as an opinionated coalition of tools for running microservices. So, Mantl is not a thing but a collection of things (Mesos, Marathon, Consul, ELK stack, etc) put together using Ansible playbooks.

Flynn is an actual product which, from what I can see, is also a collection of tools but these are written by the Flynn team themselves.

I'm happy to answer any other question you might have about it.

I worked on it for a few weeks while contracting at Cisco. At the time it was a set of tools for deploying a Mesos + Marathon cluster, and K8s was being looked at as a sub-in replacement for Marathon. I think that's now been done + much more.

There wasn't any particular magic in it, more that it was a neat package of terraform+ansible tooling for getting mesos and the surrounding ecosystem going. This was about a year ago, so things may have changed since then. I do like the idea of not inventing your own paas, but rather having a consistent framework for deploying whatever kind of paas you need on whatever cloud is available.

If you're interested in Mantl, I'd look at Mesosphere's DC/OS. It's very similar.

Disclaimer: Work for Mesosphere