Hi there. The difference really comes down to whether you are building an API or not. For example, suppose you want to serve a live feed of data via long-polling as part of your public REST interface. This cannot be done with Pusher. If you don't have a public API, and your only consumer of live data is your own website, then this distinction is not very important.
The plan is to add more push mechanisms, like webhooks (so that APIs like Facebook's can be made), and in general to ensure the system remains flexible and versatile, so that the choice of using the cloud for push is not a hindrance or a compromise.