There is a Deluxe and Premium package for both the V6 and GT. The only differences in the V6 interior and GT interior is the gauges and the center of the seat covers are a little different. (GT's have perforated leather, V6's don't. A little different design.)
Mustang V6 & GT
Deluxe - This is the base model. The interior looks like this:
There is an option on the Deluxe model called the "sport interior package" or something of that nature. It adds a leather wrapped steering wheel with silver trim on the steering wheel and the shifter, which looks like this:
Then there's the Mustang V6 & GT
Premium. This includes alots of aluminum-looking trim and leather seats. This also includes the Mycolor gauges with a display center, Shaker 500 or 1000 stereo, and more options such as the comfort package, premium trim package etc. are available.
Hope that helped you understand...