The thing about Android Auto (and I'm assuming CarPlay) is that it's not a 1:1 display of what's on your phone. It's merely an ultrasimplified phone interface.
You can only use Android Auto apps, not the entire app catalog of your phone, so it's a hell of a lot more limited than the dealer or anyone else will make it sound. If you use things like Torque, MyTracks, ArcCollector, Fuelly, WIFI Analyzer, they're not ported to the AA interface.
Android Auto is limited to things like audio apps, like Spotify/Pandora, podcasts, audiobooks, radio scanners or chat apps like Skype (why?), Kik, WhatsApp, Hangouts, etc. True utility apps aren't available. There is a native mapping interface, for folks that don't have Nav, that might be an attractive option. I think it's most attractive feature is that it reliably connects whenever my phone enters the truck, and I can use the steering wheel audio controls to start the music without ever touching the phone or even taking it out of my pocket.
Honestly, if I had a 2015 without AA/CarPlay, I'd absolutely go for the smartphone mirror over the infotainment upgrade. I'm really missing Torque and Waze, so the mirror interface is definitely on my list.
vi et armis invictus maneo..