So I recently experienced both of these, and without doubt both cause utter terror on some level. One rests on your knowledge of (typically) the past 7-8 years of study, the other, on how much Top Gear you've watched... Or rather, on your skills at precision movements with your feet whilst looking in multiple directions, changing gear and controlling a rather hefty lump of metal moving at 20-30 mph. Now I understand why cars used to be preceded by a person waving a red flag. Bring them back I say.
Since this is a blog and not a facebook post, sadly this makes it difficult to start a campaign to re-instigate said flag-bearers. I shall have to remain content with an short comparison of the two, both of which are extremely surreal experiences.
There you have it - the driving lesson is clearly worse (unless you have a real bastard of an examiner, in which case they're about equal). At least for a PhD you must have, at some point, absorbed some small level of background knowledge, if only by sheer chance from being in a university for so long. Whereas the amount Jeremy Clarkson can teach that's actually relavent to anyone at all is, if anything, even lower than one might expect.