Android Pay for the Web uses a version of the Payment Request API, I surprised by how similar it is to the Apple Pay for Web implementation. We implemented both for sending payment requests from our App, I found the Payment Request API to be much better documented and easier to implement than Apple Pay for Web and significantly easier to test.
I won't bore you with the history of the Payment Request API, but a sizable chunk of it was inspired by the native Objective-C / Swift Apple Pay API, which is why it's so similar.
With regards to Android Pay being easier to test - that's primarily because Apple Pay implements a much stricter security standard that requires a challenge/response to take place before starting the transaction.
It's actually pretty easy to shim the Apple Pay API to Payment Request. If you didn't know about this already you might kick yourself, but there's even a Google supplied shim that allows you to just write the code once using Payment Request: https://github.com/GoogleChrome/appr-wrapper
