DDG ends up working well as the default search for me. The ability to type my query and then decide if I want to use DDG or Google doesn't require me to take my hands of the keyboard. I just add a g! and I'm switched off my default. So it's a great default with an ergonomic way to switch to a different engine.

Google as default requires me to use the browser UX to change the engine, which I believe requires using the mouse or hitting tab or arrow keys. Less ergonomic.