* ownCloud is more stability-focused, used by large corporations, slower release schedule.
* Nextcloud is like more experimental, with new updates every two weeks, adding new features constantly, two major versions in front of ownCloud.
So, I tend to think them as ownCloud being kind of like Ubuntu LTS release, and Nextcloud being a regular six-month release.
They also cooperate a lot and build on top of each other (for example, Nextcloud just makes a skin on top of ownCloud's desktop client).
Also, Nextcloud's app community seems more vibrant than ownCloud's.
