Calendar Sync and Connected Calendars are both powerful features that allow you to merge your availability via Reclaim. However, they serve a different set of use cases. Here's a bit more about how they work and why you'd use one over the other.
When to use Calendar Sync
You should use Calendar Sync when you're trying to block events across calendars and prevent others from overbooking you when they look at your calendars in Google Calendar. Calendar Sync will actually create synced copies of events across calendars, and keep those copies up to date when things change.
For example, if someone uses the "Find a Time" feature in Google Calendar to find time with you on your work calendar, Calendar Sync can automatically keep your personal calendar events blocked on that calendar to prevent them from booking over personal events -- even if they don't have access to your personal calendar or have it shared with them.
Calendar Sync is also super useful if you're managing multiple calendars across clients, side gigs, or other kinds of scenarios where you're dealing with calendars that don't talk to one another.
When to use Connected Calendars
You should use Connected Calendars when you want Reclaim to consider events across multiple calendars (i.e., for scheduling Habits, Tasks, Smart 1:1s and other Reclaim-managed events) but you don't care about blocking events across calendars for the purposes of others booking time with you outside of Reclaim.
For example, you might select your personal calendar as a Connected Calendar so your Tasks and Habits get booked around both your work and personal time, but that won't stop folks from creating events on your work calendar during an event on your personal calendar, unless you also sync those events to your work calendar using Calendar Sync.
The short of it is this: if you care about being overbooked by others who try to find time with you via Google Calendar, use Calendar Sync. If you don't care about being overbooked, and you just want Reclaim to consider your availability across multiple schedules, use Connected Calendars.