Depends where you drive and how you drive. If most of the roads you travel on are pretty smooth and you're a fairly smooth/easy driver then go for 18's. But if you travel on crappy roads and/or need to go to the city/inner city alot where you encounter a fair amount of tram tracks and pot holes then I would stick to the 17's, a bit more rubber between wheel and road a little less likely to buckle if you hit something, with the stronger, more expensive wheels this isn't such an issue, but on cheaper wheels I think this is something to consider. I dare say you would get more life out of the 17" tyres too. If you're thinking about comfort, the difference would be SFA. But VVTLi seems to do ok on his 18's and he lives in the same area as you.