Please read the docs on the sync process. Marketo wins in certain cases.
Marketo will receive a flood of data from SFDC. Depending on your Marketo db, SFDC may also receive a flood. Your sync may be backlogged by several days if you do not take this into account.
- Reduce fields that will be visible to Marketo (SFDC side)
- Dedupe on both ends.
- Consider using the Do Not Sync on either side - you will have to ask Support for help. Lots of threads on that. This will further constrain who can sync over.