If using it as just a file server, owncloud is superior for its stability probably because it's tested better. I've had nextcloud break on production multiple times. It moves fast and breaks things, which is not ideal for production environments. Nextcloud has a bunch of apps, and at first I tried several and they were cool, but now I prefer reliability over a bunch of half-baked apps. I run both nc and oc by the way. Nc has better mobile app support and some cool apps, oc is frustrating because of lack of features, but it is rock solid compared to nc's frustrating breakage. Hopefully quality increases but it's been years of sometimes very painful production issues.