Hi Jeff.
Can you describe your exact test scenario? I think that would make it easier for me to make suggestions towards a testing approach.
In summary, gaedriver allows you to set up your App Engine app for testing (deployment or start-up with devappserver), while selenium/webdriver is about having an API to access web pages your app returns. If you want to test how your app renders pages, especially with JS-rich content, I would recommend using both, gaedriver and webdriver, the former to set up your app for testing, the latter to drive your web frontent tests.
cheers,
robert