I think the general case—navigating unusual terrain, intersections, traffic patterns, changes in aggressiveness tolerance, construction, adapting to "normal" problems like needing to turn left illegally at a stoplight when the oncoming lane is continual—is almost entirely unrelated to where Tesla has an advantage. This entirely autonomous general case seems to be where Google is attempting to dominate.

How much data are each of Tesla's cars really collecting vs each of Google's cars? If the data is not useful, it won't matter how many miles they've driven. Do they have the same sensor capability and collection?

Imagine the difference between, say, trying to determine a user's video preferences based upon their hit statistics on IMDB vs direct viewing data from Netflix that includes everything they've watched, how long (1 minute vs the entire show), when, the order they've watched (eg, mood predictors), frequency, etc.

I'm not saying that this is the difference between Tesla and Google, just that quantity and quality don't necessarily equate.

Not really, because by comparison Tesla cars are relatively blind. Google's cars are loaded up with all kinds of sensors collecting as much data as they can. And they have the advantage of driving back to the plant to hoover off the data with as fat a pipe as they want.

Tesla is far more limited in both what it's collecting and how it's collecting it as it needs to go over a cell signal. For example they are using a camera for a hefty part of the autopilot system. They obviously can't stream every frame of that camera up to their servers for deeper analysis. Besides being infeasible bandwidth requirements it would be an insane privacy violation of the owner.

Does Tesla collect data when Autopilot is turned off? Otherwise I would be very curious to know how many miles of non-highway driving it has logged.

Data is a commodity.