It's basically the same nip46 server, but nsecbunker server also supports some custom admin RPC so that nsecbunker.com UI could talk to any nsecbunker server. Plus limited access to UI - assign admin pubkeys, those can log in to UI using nip07 to control the keys.

Nsec.app hosted has no UI access control for now, and no way to access your hosted instance from external network.