You might also be interested in a talk I gave at the Kubernetes launch in August about our work coming the two: https://youtu.be/aXcdHwQ5GgQ
a1r is exactly right - Mesos does great in virtualizing your data center, Kubernetes is a framework on top of that.
Here's a blog discussing the current state of K8s scale: http://blog.kubernetes.io/2015/09/kubernetes-performance-mea...
Note that Bob Wise at Samsung has been driving some horizontal scale testing, and has got a 1000-node cluster up and running, so that's a current "best case" scale number.
(work at CoreOS)
I'm not sure what the trade-offs are with running Kubernetes on Mesos - that would make for an interesting article.
Running workload-specific schedulers like Kubernetes on Mesos is one of its fundamental ideas. From the paper:
"It seems clear that new cluster computing frameworks will continue to emerge, and that no framework will be optimal for all applications. Therefore, organizations will want to run multiple frameworks in the same cluster, picking the best one for each application."
https://people.csail.mit.edu/matei/papers/2011/nsdi_mesos.pd...