Honestly, I wish I had the 12" over the 8.4". The few times I've had my truck in the shop for warranty work (the last time was to replace the transmission, THAT took 5 weeks due to "shortage in the supply chain"), I was given a fully loaded Durango. While it didn't have the 12", it did have a larger screen (not sure of the size, but im guesing it was around 10" at least) than what I have, and the difference was VERY noticeable!
Especially when using the Nav system! Plus add in a few other "bells and whistles" that my system didn't have.
When I got my truck back, my screen al of a sudden looked ...lessor. I thought that someone had swapped out what I had and put in a smaller screen! That was how noticeable the difference was!!
If I could do it (I know it can be done (LOTS of YouTube videos on it), it's the cost that turns my gut!), I would swap out mine for the 12" in a heart beat!!