Buying from Amazon is easier because it just becomes part of your Kindle library. The price is the same either way.
The difference comes in what you get in our packages, which include the Kindle version, plus the epub version in case you switch devices later, plus the PDF.
It's a little more hassle with our files, at least the first time until you get the hang of it. So... ease vs extra value is the difference, and which is most important to you would be how to decide.