Tough choice you have!
As far as I know, you can go as wide as 215/35/18, but you can fit 225/40 on 17"s an the bigger sidewall would give better traction and better grip on bumpy/ rough surfaces!
Dan
Im actually lookin for a smaller set of wheels for my motor.... Although it is comfortable and all on the 18's I just think i prefer the look of a slightly smaller rim with a bit more sidewall on the tyre and as Dan pointed out there I can go that little bit wider on the foot print then too.....