Personally, Ive had 4 diff sets of wheels on my car since Ive had it. 18in DD Bullitts, then 17" Cobra R's, then 17in DD Bullitts, then I finally landed on my Saleens and Im happy with the them.
Personally i think a 17 looks great on any SN car but 18's just seem to fill out the wheel well better.
Here's a little comparo from DD 17" Bullitts and my 18" Saleens. Ignore the dirty car, its been sitting in the shop for a month and a half, bout got the motor swap finished (workin 50+ hrs a week and keepin the woman happy takes away from car time haha).
Personally I just think 18's look a lil better on these cars. And my Saleens are polished and don't really seem to be any heavier than my Bullitts were. I didnt weigh em though.