Hi Linus, interesting use case here. You mention leads and contacts; are these coming from a previous SFDC instance, and/or another CRM entirely?
One thing to keep in mind is how Marketo resolves duplicate records. Basically, its designed to respect Salesforce's database; this means records that exist in both Marketo and Salesforce, before the sync, will not necessarily deduplicate when you sync.
Given this, I'd recommend integrating before loading the data, and then using Marketo to pass the data to Salesforce, as much as you can.
Interested to hear if others have more insights to offer. For example, managing the contact data can get complicated as Marketo resolves SFDC lead and contact objects into one group.