I don't know the weights of the two, but with only 1" difference in diameter, i'd bet my week's earnings (i have no job now ) that the 17" are quite a bit better. What year stang, the 16's are aluminum and not steel, right? I'm pretty sure Ford only made steel 15" wheels from 94 to about 96 and not 16". if aluminum, you're good to go i'd say.
Steve you need to get more power and stop worrying about putting your stang on a diet. Weight reductions are great but you need the power first man. No body cares if your car can run 13s if you dont have any interior, or doors on the sides. Get those 18s you wanted, your nitrous, that way you willlook cool and be FAST.