Correct me if I'm wrong but Ganeti only does Xen VMs while ProxMox also does LXC containers. Nice to have both via one central UI.
I've used Ganeti Web Manager[0] in the past, but would probably try out Ganetimgr[1] for future installs. The command-line tools are very good however, and I don't think the web interfaces expose the full functionality (i.e. rebalancing, node evacuation, master-failover, etc).