What platform is your organization using for your customer user database?

We'd like to expand all of our product and company communication to our entire user database vs. customers that we're in contact with. This will add 300K+ contacts to our mailable database, and I wanted to know where other people are managing this. Right now the database lives in a stand alone system but we have the ability to either sync it with Salesforce, Marketo or any other system.

After talking with our Salesforce Admin, we ruled out SFDC because we didn't want to clutter up customer accounts for our success managers. There isn't a way to bring them into the system and hide them from the account view.

Now we're considering just adding them to Marketo in order to include the users in relevant email communication.

How are other people successfully doing this? What are some easy things to overlook that we should think about before doing the sync?