So yeah, you have to really look at the weight and how it looks with the body style you have. My opinion is that the new edge looks stangs look great with 18s, SN95 look better with 17s (17x10 rear) with the exception being the Saleen style in 18s look great on both.
It really won't matter with the gears but you will have some "slower" speeds with the 18" wheels. When I went from my 17" Cobra R's to my 18" Saleens I could tell a difference in acceleration. It was a little bit slower.
I would say get what you like. 18's aren't too big at all.
I also noticed a slight decline in acceleration with my 18's as opposed to the stock bullitts. Between the larger tires and larger rims, a lot of rotating mass was added. Still though, i'm glad i went with 18's, they make the car look so much better.