Salesforce Platform License: Designed for users who need access to custom apps but not to standard CRM functionality. Users with this user license are entitled to use custom apps developed in your organization or installed from Force.com AppExchange. In addition, they are entitled to use core platform functionality such as accounts, contacts, reports, dashboards, documents, and custom tabs. However, these users are not entitled to some user permissions and standard apps, including standard tabs and objects such as forecasts and opportunities.
Salesforce Chatter Only - These users can access standard Chatter people, profiles, groups, and files, plus they can: View Salesforce accounts and contacts Use Salesforce CRM Content, Ideas, and Answers Modify up to ten custom objects
I must be missing something but I don't see the difference between these two sets of functionality.