It depends on what you want - you didn't give us any details or requirements.
A 17X9 will cost a lot less in terms of both the rims AND the tires. You will also be much less likely to have any rubbing issues. A 17X9 will fit a 255/40-17 beautifully without rubbing or modifications while staying cheap. An 18 will look a little better due to being bigger, so it depends on your preferences.
The AutoGuide.com network consists of the largest network of enthusiast-owned enthusiast-operated automotive communities.
AutoGuide.com provides the latest car reviews, auto show coverage, new car prices, and automotive news. The AutoGuide network operates more than 100 automotive forums where our users consult peers for shopping information and advice, and share opinions as a community.