I don't know who is bigger. But, I sold 4 cars this past year and listed them on both AutoTrader and Car.com. The ultimate buyers were two from AutoTrader and two from Cars.com. I also had about an equal amount of inquiries from both.... maybe a little more fro AutoTrader.
So, I'd say, based on my experience, that looking or advertising in both is beneificial.
-Wick