In your position I'd start by talking to customers and understanding their requirements. Unfortunately, although Apache Mesos + Marathon is generally liked more and simpler, it's still a commercially owned space when you run into trouble. On the other hand Kubernetes is more open and popular (check search results, about 10X) and customers like that they can move around among GKE and AWS and other Kubernetes environments. So although I think Kubernetes is overly complex for the newcomer, (and you may need Helm as well) it's going to be more popular with your customers. Also, Kubernetes is fairly open in terms of extensibility (esp if you work in Go lang).
More thorough discussions here:
* https://logz.io/blog/kubernetes-vs-mesos/
* https://www.stratoscale.com/blog/kubernetes/kubernetes-vs-mesos-architects-perspective/
