I was asked recently what XML, XSD and XSLT editor I use. Given that I am an independent/small consulting firm owner, spending $1000s of dollars on development tooling that will come out of my own pocket is not my first choice.
The short answer is that I bought a license for the oXygen XML Editor. The price is very reasonable. I’ve used it in routine DataPower development and troubleshooting for a couple of years now. The functionality for comparing XML files works great. I’ve used the XSL Debugger numerous times. It doesn’t provide support for the DataPower Extension Functions and Elements, of course, but a lot of work can be accomplished before those are added in most cases.