For some history: ownCloud is a very old stable product. 2-3 years back it got forked into Nexcloud because the founder of ownCloud had some disagreement about ownCloud's direction.
Both the products are backed by companies and both are doing quite well. I would say Nextcloud goes more and more into expanding it's use case and thus makes it product more extensible via plugins. This can be good or bad depending on how you look at it. Plugins go unmaintained/incompatible over time and are a constant source of pain when upgrading. Wordpress gets away with this because it has a massive community.
ownCloud on the other hand has decided to double down on it's roots of file sharing/syncing. I heard they rewrote their stack from PHP to Go now and the frontend is now React.
Both the products are backed by companies and both are doing quite well. I would say Nextcloud goes more and more into expanding it's use case and thus makes it product more extensible via plugins. This can be good or bad depending on how you look at it. Plugins go unmaintained/incompatible over time and are a constant source of pain when upgrading. Wordpress gets away with this because it has a massive community.
ownCloud on the other hand has decided to double down on it's roots of file sharing/syncing. I heard they rewrote their stack from PHP to Go now and the frontend is now React.