What is the best way to find the delta between 2 versions of a process model?...

One idea is, you can verify the process model's XML file for different versions using diff tools. You can get to the xml versions of your model by browsing to your Appian installation directory - file store location (by default _admin directory) and then going to the models folder.
Other idea could be, visually check the versions of the process model.

You are going to get very little from xml comparison. The files are huge and the order in which objects appear can change from export to export (even with the same version). Your best bet is to document different versions of your model - preemptive measure. At this point, visual inspection will be the most effective.

You can use Generate Documentation for the different versions as another method of visual inspection. forum.appian.com/.../Generating_Process_Model_Documentation

Htmldiff on the generated documentation files can point out some basic changes. Not very good but better than nothing.

thank you everyone for your suggestions - I had already tried visual inspection and generating the documentation. I'll give the xml diff a go now to see if it highlights anything.