I'm happy to see this thread as I was wondering the same before I picked up my Platinum... Sync3 / Android Auto was one of the big reasons I held out for a 2017. I've had it a couple weeks now so I'll share for those of you that are waiting patiently for your orders to be delivered...
Bottom line.. I'm glad I waited for the 2017!!! Sorry to all of you with a brand new 2016, but the rebates on the 16s should help you feel a little better!
I can confirm Android Auto only works when plugged into USB, not over bluetooth connection. AA will disable your nav & phone screens when running, so this actually makes it simpler to choose when you're using AA or Sync, otherwise you would have to disable AA if you didn't want to use it every time you get in the car. I agree with the previous posts that Google maps is a must for long haul trips and is superior to the built in nav for traffic alerts and rerouting. I had the Gen 1 Sync with built in Nav in my 09 Explorer and I would often use my phone when traveling for traffic/routing as navigation around construction and traffic is superior with Google - even with Sirius Travel Link. Just driving around town though, I've been happy to not fuss with plugging in my phone and just use Sync as Sync 3 is pretty solid on its own.
Pandora works great both with AA, or over bluetooth with Sync. Pandora applink works great through Sync giving you all information, including album art, on the screen. You also get the same with AA, but the screens do look different including color, contrast, design layout etc..
The phone and text messaging also both work great with both AA or Sync. AA is definitely designed to be controlled by voice as it gives you far less information and menu options on the screen. For example, you can send and receive text messages all through voice control including the ability to send customized messages. With Sync, you can view message information on the screen (who it is from) and a button to "hear" messages received while car is in motion, but you can not send messages until you have stopped moving. Messages you send with Sync have to come from a list of preselected statements such as "Be there in 10 minutes", "Yes", "Thank you", etc. If you try to use Sync voice to send a message, it responds "I'm not able to send messages yet." (I have a Galaxy S7, so this may be different with another phone...?) Using the voice menus to call in both Sync and AA are very similar. Again, the phone screen has much less functionality than the phone screen on Sync. For example, you cannot browse your contact list, just recent calls. The same is true with music, as you only have access to your audio listening choices on your phone, not the radio/CD/Sat. You have to exit to Sync if you want to control the AM/FM/Sat stations (however you do retain some control through the steering wheel controls and secondary display in the instrument cluster).
With Android Auto, the menu options along the bottom of the screen change to Nav, Phone, Home, Music, Exit to Sync. One minor irritation is when you click the exit AA on the menu, it brings up a second screen that spells out "Exit to Sync" that you have to click a second time. I suppose this prevents an unintended exit but if you just want to jump to the radio or climate controls, it adds an extra step.
When you have your phone with AA connected USB, the system will automatically jump to the AA home screen which is the Google Now cards trimmed down to travel, music and calendar info etc. I'm not sure what would display if you don't have Google Now, that may be something Google forces you to activate to use AA?? If you exit to sync and your phone is still connected USB, the phone, nav and Apps menu buttons will all take you back into AA. (The apps menu button actually changes to an Android Auto button.)
There are a lot more apps compatible with AA than with Sync, but most of them are music and messaging apps. I did try Glympse which integrates with Sync. It works pretty well and lets you share your location with others. When they view your location, Sync announces it to you and you can monitor on the screen who is "watching" you. Kinda creepy, and cool at the same time.
That's a long winded run down on what I've tried so far. Since I tend to keep my vehicles for several years, it is nice to know that I'm a little more future proof with Sync 3 and AA/Car Play going forward. Who knew when we were kids that one day we really would be talking to our cars, and they'd be talking back to us... Kit.