I've been on my 18's since June and I haven't regretted it at all. I live up in Northern Westchester and drive through the Bronx, over some very hairy expansion joints and potholes, to NYC every day. I've hit some monsters and was sure that I bent one, but haven't suffered a scratch yet. The ride is definitely harsher on 18's. It's noticeable when I hit some of those joints and think that my dashboard is going to explode, but it hasn't been harsh enough to make make me regret them, even for a second. I had the exact same concerns as you when I was choosing my wheels. I suggest the 18's for sure, you'll wish you went bigger if you go with the 17's. I know I would have.
Just make sure you get a quality wheel that can handle some abuse, not some junky 18's made of a softer metal. I'm on OZ Superleggeras and have loved every minute of 'em.