Having had the 12" screen for over 2 years I would not be unhappy with the 8.4". The split screen with AA/Carplay is meh, the ability to use the whole 12" screen is meh. It looks amazing, it makes people go "wow", but it's not a necessity. A lot of times I end up with Spotify through AA on the top half and "music" on the bottom half because I'm too lazy to pick another option on the bottom. There is no way to get music up top and nav below, and you have to look REALLY far down to see the bottom. That's a long way to take your eyes from the road.
I have the 12" screen, and agree it's not a necessity, but it is nice and certainly adds some wow factor. Wow factor is always nice to have when you eventually sell the truck, so that's something to consider.
When I bought my truck I was actually looking at one with the 8.4 instead. My wife wanted this truck (it had more features) so I got it. Now that I've had the 12, if I was ordering a truck, I'd get it again. I may get a 8.4 again if I was buying off the lot, but to special order I'd probably spend the extra.
And no, I've had no issues with the radio. Actually I've had more issues with the 8.4 in my wife's Jeep, because it occasionally won't display the guide lines on the back up camera. As far as I know they all use the same software so I don't think it's an issue exclusive to either radio.
As for using the dual screen (aka "cards"). You can absolutely have music up top, and nav on the bottom (2 options for that actually). You can also put nav on top if you don't want to look down so far.
You can either press the "map" button on your media display to get a half map. This doesn't work on all media sources though (such as SXM).
You can also go to the home screen with two customizable cards and select your music source on one and your nav on the other. Notice the little dots near the top of each card? Those indicate you can swipe right/left on that card to change what it's displaying. For example the Travel Link car can show weather, fuel prices or I think some other options.
Edit 3/24 @ 1059:
Remove and repost images with road names and address redacted.