I'm not an SFDC admin, so this is a bit out of my area of expertise, but it sounds like a task to me. When the person fills out the Marketo form, it will search the Marketo database for a record with an email address that matches the email address submitted with the form, and then updates that record. I would advise against creating a lead as this would give you problems with duplicates (multiple records with the same email address).
Whether you want to assign the task to the lead owner or account owner depends on your business practices in Salesforce and isn't something which has a general best practice. I would ask myself "who will be filling out this form?" and then "does it make more sense for their owner, or for the account owner, to follow up with them?"