Hi @katherineladue,
If your client says that it’s fine to overwrite existing information, assuming that the ZoomInfo data will be more accurate, then I would probably go with that decision. You can always export all contacts and companies before turning on the sync, as sort of a backup, in case something goes catastrophically wrong unexpectedly.
Creating duplicate properties might be a good idea for some of the fields that are critical to business processes (and not just informative) and used in central automation. If you for example have workflows for routing contacts based on industry, it might make sense to create a duplicate property first, check the information that is flowing in, create a report to compare your own and the ZoomInfo data before deciding to switch over entirely. I wouldn’t do this for all properties but for those where new or wrong information could ripple through your portal.
Regarding the mapping: Use good judgement and map company attributes to the company object in HubSpot and contact attributes to contacts. I don’t see a reason to do this differently and you can always create a sync property later should you really find that something needs to be displayed on a contact record, for example.
Hope this helps!