Key question is what you're actually looking to use it for?
NextCloud is more or less a "local cloud storage solution", in that it runs as a server and then there are client apps that go on your devices and the two sync up. There are ways and means to expand NC with various other stuff like calendar management, team collaboration and other bits, but underneath it's essentially your own cloud (hence the name of the project it forked from, OwnCloud).
OMV on the other hand is a NAS/server solution, again with drop-in plugins for stuff like Plex servers and web servers. You just connect to it over the network as you would any other server, without a specific local client.
Personally I've run both over the years, but blew away the OMV device and replaced it by a DietPi-based Pi and the original tasks are now shared between that and a second Pi running Docker via HypriotOS. My NextCloud server sits on the latter (although the former can support such an install too) and it's reasonably reliable and stable.