From the description and screen grabs on the App Store, I think PhatPad is a complete note taking app: you can add images and text to a note and also sketches. It also has shape recognition so that you can hand draw a circle or a square and it will transform it into a geometric shape. It seems similar in functionalities to Notes Plus (but probably not as rich).
On the other hand, WritePad is mainly an input device. You can of course store and organize your notes inside the app, but they are only text notes. The main function of WritePad is to produce text through handwriting recognition.