With the new Client Alerts technology available, should I use Client Alerts or Platform Notifications in my application?
First, it's important to note that a given eBay application (AppID) can support Client Alerts or Platform Notifications, but not both. This is because back-end routing procedures send the event to either Client Alerts or Platform Notifications.
So if you have an existing application which uses Platform Notifications, you should evaluate whether moving to Client Alerts will be beneficial or not.
However, if you're developing a new application, the decision to use Platform Notifications or Client Alerts is largely determined by the "physical" makeup of your application. For example, if you have a known server which can receive events, choosing Platform Notifications may be the best bet.
However, if you're creating a widget which may be distributed to any number of arbitrary IP addresses, then Client Alerts may be better. In other words, Platform Notifications require a known end point (ex. HTTP server or email address) to receive notifications. Client alerts allow your application to poll for events from any arbitrary device.