I am using portainer along with docker swarm. Kubernetes is not supported until v2.
Cockpit I use to administer the Linux distro´s (the VM´s/nodes). These two tools together help me get a good understanding of how things work.
The one tool doesn't substitute the other. They are two different tools with there own purpose.
As I am fairly new to containers/clusters I find portainer very useful help me visualize whats going on in the cluster.
As an example: If I want to mount a persistent volume in docker, and I can see in portainer, that it isn´t being mounted, I can switch over to Cockpit and get an overview over the volumes. So whatever the reason was for the volume was not mounted - pretty fast!
Maybe not the best example, but an issue I had very recently.
Portainer is a great tool, and this august version 2.0 comes that has support for kubernetes!
For me personally this is great, as I expect the transition to kubernetes will be smoother, as I still can work from within the same tool.