What is difference between Protocol Adapter and Edge SDK for Gateway?

I'm not super familiar with the protocol adapter sdk, but it looks like the main difference is that it listens on a port for incoming messages  rather than polling the devices like the Edge SDK would do. So, if you have a device that is publishing its properties via some protocol, you can use the protocol adapter to listen for those messages and then parse them into Thingworx properties. Alternatively, if your device is not publishing its properties, you can use the Edge SDK to  poll the device periodically and then parse those into Thingworx properties.

Both act as gateways and both communicate to Thingworx over the Always On  Protocol.