IMHO that depends on couple of factors and use case, and I am happily running either on two different NASes.

OMV: -- has Docker plugin with Compose support (no need for sep. Docker GUI like Portainer) -- SMB shares are (somehow) more reliable on Win clients -- has more beginner friendly GUI and attitude, easier to share with other users -- batteries-included features like fail2ban + Wireguard

Cockpit: -- first class citizen on EL / Fedora distros -- Podman yes, Docker no - no Compose/Quadlet support -- killer features like VM managements and Terminal -- bugs with Samba