Guys, today I did side-by-side testing at a dealership with and without my iPhone XS Max pushing CarPlay. For those interested in this subject I had 5 final observations, and reached a conclusion for me.
1. Going back and forth between them, the 8.4 inch diagonal screen actually looks slightly wider in CarPlay mode (in other words wider horizontally) than the 12.1 inch diagonal screen. Would that even make sense mathematically? I don't know, but that was a small difference I felt I was seeing.
2. CarPlay running on the 8.4 inch uconnect sits higher on the dash than when running on the 12.1 uconnect. The difference was significant enough to note.
3. Something that surprised me: going back and forth I kept seeing that hands down the 8.4 inch screen was the better quality display on the metrics that display people usually measure. The 12.1 has all kinds of light leakage from the backlight. Also it's blacks aren't as inky as the 8.4, and it's colors aren't as vibrant. Frankly, and I hope you guys would extend me some grace as I don't mean to offend, but it shows like some of the lower quality laptop displays. I would also not be surprised if the 8.4 had higher pixel per inch density - it looked a notch more crispy.
4. One functionality I definitely preferred about the 12.1 was the ability to have Waze on top, media on the bottom.
5. One functionality I definitely preferred about the 8.4 was that my wife could turn on her seat heater (or cooler!) without having to mess with the screen.
Anyway, for me I've decided to go with the 8.4. Appreciate you guys and the info this forum provides.
Don