Since asking this question I have tried both GUT and WAT and have settled on WAT.
The key differences I noticed are:
- GUT shows the test results in a separate game window
- WAT shows the test results in a panel that's integrated into the Godot editor
- GUT has a VSCode extension to run the tests
- Both GUT and WAT have the ability to run tests from the command line
- WAT has a C# version
Either will work fine for any GDScript project, I just chose WAT because I liked the integrated UI and I needed C# support.