I too am looking at an 08. I was lucky enough to drive both the 6 and 8. I personally after driving both would not get the 6. It does have acceleration power, but the real difference in the two is evident in the torque. On the V8, when you drop the accel you notice the power difference immediatly. The V6 seemed to take forever to spool up, the V8 downshifted and picked up right away.
Now take my opinion with a grain of salt because I don't drive underpowered cars, my daily driver is supercharged so I'm used to the power. When I drove the V6 it left me needing more. Couple that with the negligable gas mileage and negligable price difference and the decision is easy. Get the V8 if you can find one.
Also I was looking specifically for a ST with no sunroof, so I was searching Fords inventory in my 100 mile radius. Not one single truck had the nav and I probably looked at 60 trucks online. Hell, it's hard to find one without a sunroof either.
Good luck, let us know what you do. I'm making my decision tomorrow as the truck I found had everything but the nav and sunroof, the sticker is a little high for my intended use, but the bells and whistles will be very nice.