Alyssa, you can log into SFDC with the Marketo Sync user account, especially if you will use it sparingly, but keep in mind that you will have the same profile as the Marketo Sync user and will not be able to access any of the fields that are not visible to that profile. We use Scenario 1 because my user account is associated with the System Administrator profile. Scenario 2 would be the best as long as there is no compelling reason to differentiate between the marketing users regarding permissions, lead ownership, receiving notifications from SFDC, tracking changes made to SFDC, etc.