Best Practice on Dedicated Salesforce License

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.

We currenty have Scenario 2 and it has worked well for us. I think Elliott mentioned the pros and cons very well.

Ideally you'd give everyone in the marketing department at least read-only access to SFDC. It might be worth it to pick up a phone and call your account manager to ask if they would allow you to add any Contact manager licenses to your account (much cheaper than pro or enterprise, but also more limited in that they can read contacts & accounts but can't see campaigns).  If that won't work then I'd try to do #1, because I find it very useful to have the Marketo user have its own account. That way, it is easier to deduce that something unexpected that  you're seeing on a lead/contact record, was actually changed by Marketo itself - and you can clearly rule out your own manual one off changes to leads/contacts. 

We use Scenario #1. I have my own user license that is set fairly high in terms of security and data access where the rest of our marketing team only requires some basic data access and mostly read-only access. Scenario 2 could work and you could build out another user license if you find there is the need for a marketing member to have more data and admin priviledges.

We're a bit a hybrid of your scenarios. We have an SFDC user set up for system integrations anyway (SFDCAdmin), so that user is hooked into Marketo. I have an admin account as our primary SFDC Admin, our Sr. VP, Marketing  has a near-full-admin account, and the remainder of the department shares a general Marketing SFDC login. The Marketing login has permission to view or edit pretty much any record, but is locked down from any admin functions.

There is no right and wrong answer. Just think about who needs to access what, who truly needsa a dedicated account for tracking and reporting, and go from there.