I'm going with 20's. Putting on Armano Hampton style 20 X 8.5 with P255 35 series ZR Goodyears sized to match stock diamter. Then going with
Eibach pro-kit to close up the wheelwell gap. Getting wheels/tires in put on in the morning, and Eibachs next week.
(web site to see wheel is
www.discounttire.com/....choose 2005 mustang gt, oversized options)
IMO, Hampton is a nice compromise between the old 5 spoke style and the current trend of larger chrome wheel and tire combo. I predict that over time, the larger wheels will be viewed as the way to go. Remember when 15" wheels were it? Now we think 17's are too small. Wont be long and 20's will be the norm on performance cars.
In the real world as a daily driver, the penalty associated with having to overcome a larger rotational inertia on the 20"diameter tire is not a big deal. Maybe a couple of tenths in the quarter. And for 0 to 60 sprints, hooking up the tires is a bigger problem than initially accelerating the mass associated with the wheels and tires. Mass of a 20" wheel only cuts into your accelation after you are fully hooked up. Unsprung weight will also affect
suspension response, but again, is this a race car or daily driver? If a daily driver, I doubt if the effects are significant relative to what is encountered on a regular basis.
To me, as a daily driver, I am rating looks of a 20" over the performance penalty. Others may differ in opinion, but thats all part of what makes life interesting.