I have no experience with Sandstorm so I can't fairly compare the two but I can say I am very satisfied with Nextcloud + Collabora CODE for file sharing, calendar management, and document editing. As far as chat goes, Nextcloud Talk is supposed to be very good but I've never done more than a few quick tests with it. If you have an XMPP account or if you can set up a server (I recommend Prosody), there is an XMPP app for Nextcloud.