I've never actually used Android Auto, but use CarPlay frequently. What do you like better?
All of my stuff at home is integrated with Google Home products, and that won't change no matter what OS I use - because thankfully Google makes it pretty easy to use on Android and iOS. My family aside from my wife all use Apple stuff - so it would make sense to stick with it. And my wife is an avid Google Photos user so we share everything that way and it works very well for us. I don't like Siri - but I also wonder if I would feel the same way about Assistant. I like asking the occasional question, but not for all my tasks. I still like setting most of my own tasks, alarms, etc.
Android Auto's layout is less efficient than CarPlay. CarPlay mimics the classic iOS home screen in most regards, including big ole icons for apps. It makes navigation by touch screen much easier. CarPlay also includes the 3 most recently used app icons on a strip on the left hand side of the screen, making a quick switch between two apps (ie. Maps to Pocket Casts and vice-versa) a snap. CarPlay also lets you add/remove/rearrange apps for the display layout, while Android Auto surprisingly offers no similar customization.
Conversely, navigating around Android Auto takes more touches for the same actions, respectively. The home screen either shows upcoming appointment destinations, currently playing audio, or nothing at all--basically a stripped down fixed notification shade. Navigation requires multiple touches to smaller targets along the bottom of the screen. For instance, to get play audio, you have to touch the small headphones icon which then brings up a list of all available apps, listed alphabetically. You then likely have to scroll to get to the app. Again, cannot change this list. Furthermore, messaging apps also live in this 'audio' submenu. If you want to switch apps, you have to first hit the center home icon, then either touch a shortcut on home screen if available (navigation or audio playing appears here) or once again plow through the drop down menu from the bottom icons.
Google Play Music offers a more robust interface on the CarPlay version--go figure.
Two areas where Android Auto has advantages, IMO--navigation with something other than Apple Maps and a few more available apps (i.e. Telegram messenger). The former has proven to be less troublesome than I expected--Apple Maps has worked quite well in my area. It falls short against Waze and Google Maps in being able to navigate to multiple stops simultaneously but otherwise has worked just fine. The latter I find to be even less of an issue as all of the most used apps (Pocket Casts, GPM, Spotify, TuneIn, Audible, WhatsApp, Pandora, Amazon Music, etc) exist on both platforms.
The other area where I'd normally give Android the clear win, voice assistants, is also not as impactful in the automobile, in my brief experience. I find Siri to do just fine and reasonably accurate for tasks (messaging, making calls, creating reminders, seeking navigation directions), on par with doing the same things using the Assistant on Android. Google Assistant pulls away when it comes to anything where searching the web is required.
CarPlay
Android Auto