Remember Sat Navs become part of the driving test from December
In regards to the original question, I've always had Android devices so I'm biased towards Android Auto.
One thing to point out is that both are technically designed to be a car-friendly interface and be shown on a built in display driven by a wired connection to your phone.
However Google added the option to allow Android Auto to run on the phone itself if you don't have a compatible system - but as discussed it doesn't seem to be completely the same. This on-phone version can be triggered by connecting to a Bluetooth device alone rather than the physical USB connection as well so is a bit of a hybrid solution - I think full Android Auto still needs the wired connection at the moment.
Apple have been working on a wireless version of CarPlay that uses a WiFi system in a car (due to the bandwidth needed) but it doesn't seem to be widely supported yet, and Google have mentioned they are also working on a fully wireless Android Auto system, but there hasn't even been a demo of that yet.