The answer is neither: both tires transmit the same power to the ground. It's just the spinning tire that's burning off the excess power in the form of friction. I've gotten into many heated debates over this and I decided I needed to find out how I could prove that I was right. I asked the question on
The Mythbusters Forum
I was called all sorts of names basically telling me I didn't know anything about anything and only 2 people in the thread actually took the time to analyze it and after doing so, they came up with my conclusion. You'll notice the thread is under moderation right now so I'm assuming we'll be seeing this myth on the show eventually.
I suggested using a drive-on alignment lift since it has bearing plates. Tie the car down like it's on a dyno, drive wheels on the bearing plates and attach the plates to scales(like they used when they tested the myth: Does a fwd car have more traction in reverse?).
It was funny how stupid some of their responses were including all of their formulas and such, trying to make me feel stupid but I was having the last laugh every time.