The great thing about the Base 370 is that you still get the premium interior sans the seats, but even then the base sport seats in the Z are great. The base Mustang is VERY plain and they don't use the good sport/cloth fabric like the Z does...
The Premium is on par with the Touring Z, but the base is just blah. I could NEVER own a base Mustang. The only time I'd buy a base model is if I was planning on tearing it down to turn into a track machine.
But my Premium package 5.0 is super comfy and the steering wheel, seats, and other leather appointments are fab.