what class of autox?
i voted 17. they are much better in terms of performance. the only reason to go with 18s in this case would be if you were in stock class and you wanted the extra inch of wheel width.
if you are serious about autox, come visit the autox forum. we'll set you straight.
for auto-x: '04 Liquid Grey ZX3
for street: '02 Infrared SVT