Comparative between NextCloud and Webtop? It’s not easy to made, NextCloud email app is quite young as for I know. Webtop’s one is very mature and several features
On the other hand NextCloud’s file sync/cloud is far better than WebTop (I know that there will be an integration in the future)
In my case I use both, Webtop for email/calendar/contacts and NextCloud for cloud and files management.