I had been waffling several times with respect to the screen. Ended up with the 15 / Sync4a setup. I have rather enjoyed it. Quick, very responsive to input. My hesitation was physical button/dials for controls vs being touchscreen based. However, I realized coming from a Ford Edge sport with Sync3, this was a touch-based setup before and was already used to this.
Would recommend running the unit in "Dark mode" -- It will auto-swap display mode based upon outside light conditions, but the white background is just too bright for my liking, even during the day and after adjusting the brightness of the screen. Been super happy with dark mode.
You can enlarge the navigation to occupy about 2/3 the screen if needed (at least the nav via carplay) which is great.
The downside to the screen thus far have been: 1. The camera is not readily accessible (it is a series of 2 touches in different screen locations to get to it -- IMO should be a button on the home screen, maybe a selectable item in "row 2", or default to that when pressing the expedition icon) 2. It sure is a fingerprint magnet. I need to figure out a solution to this (will likely carry a cleaning cloth?)
Overall, I have been very happy with the system. Would I be bummed with the 12 inch screen? Absolutely not, that screen is awesome too. I would not make a decision of which vehicle to get based upon the screen. Both are great.