I'm getting the in-dash nav because not once in my life did I ever regret spending the extra money to get every available feature on something several years down the line, but I regretted NOT spending the money every single time.
That goes for cars, phones, TVs and computers. I don't apply this to appliances like refrigerators, stoves, washing machines or dishwashers. I went as far as to get an external ice maker on the fridge, but none of the computery electronic foofaraw.
I like the idea of having something that just works, without worrying about getting one computer device to talk to another, or having to put a bunch of ugly, Mickey Mouse mounts and wires dangling over my dash. MFT might let your nav app work through the big 8" screen, but that's just a potential place for software incompatibility to rear its ugly head. Especially a couple of years down the road.
That's not to mention that if you're depending on phone nav, and you don't have one of the pricey nav apps that downloads all the maps to your phone, you can lose navigation just when you need it most. I can think of at least ten times that's happened to me, and there are probably more. As good as Verizon coverage is, in hilly Los Angeles there are a lot of dead and "no data" zones.
My $.02.