I'll echo what a lot of people have said here - fields are important. A few things to consider: Standard Marketo and Salesforce fields will sync during the integration, but custom fields won't - are there important fields in Marketo that will also be available in Salesforce? Those Marketo fields should be replaced with the Salesforce fields and the data migrated over. Lead and Contact fields with matching API names will map over to a single field in Marketo, but those with different API names won't (even if their friendly names match). Support can map these latter fields together for you, but they can't if used in any assets, so plan time into your project to take that into account. Don't sync over every field on the Lead, Contact, Account, or Opportunity objects - just sync those you'll need to edit/reference in Marketo. This will ensure a faster sync between the two systems - why sync data you don't need? It's quick to setup the integration, but be sure both systems are in the ready to do so.
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