What are the Differences Between Apple CarPlay and Android Auto?

Apple Carplay 

Make phone calls, access music, send and receive messages, get directions optimised for traffic conditions, and use Siri Eyes Free

Further information can be found here: http://www.apple.com/uk/ios/carplay/.

Android Auto

Make use of apps and services including Google Search, Google Maps and Google Play

Further information can be found here: https://www.android.com/intl/en_uk/auto/.