I'm not sure this is exactly what you are looking for but I was looking at it on Apple's site just the other day. It may be helpful to you.
[URL="https://www.apple.com/ipad/compare/#mn_p]Compare iPad models[/URL]
This article talks about the differences between the 2017 and 2018 models. And this article compares the original iPad Pro 12.9 with the 2017 version.