"View in calendar" is for Exchange accounts, where the app (AquaMail) syncs calendar invites into system Calendar -- and it opens the synced event there, that is, the actual synced event.
".ics" -> View creates a standard format .ics file and asks Android to let you pick and app to open it with. This is "disconnected" from the synced event.
About Informant -- please search the forum, I vaguely recall some issues with that app.
