They look to have almost the same functionality if you ask me.
With iOS 8 and using "Hey Siri" you never have to touch a button to get access to voice controls. It's not clear from the demo if "OK Google" works in a similar fashion. I'm assuming it doesn't or they would have shown that off rather than having the guy press the voice button on the steering wheel to interact with AndroidAugo. T
Another differentiating factor seems to be that there is a pretty big difference in the supported vehicles. Apple's CarPlay is supported by fewer car manufacturers, but more high end models (Mercedes, BMW, Jaguar, Ferrari). AndroidAuto is supported by more car manufacturers overall, but mostly of the common type such as Dodge/Chrysler and the like. I also see no mention of AndroidAuto being supported by aftermarket systems by Pioneer or Alpine. No matter what kind of car you have, if you install one of those systems you can have CarPlay.
It just seems to me to be another Google rip-off of an Apple feature. At least Apple won't be selling my location or driving habits to advertisers.