I would go either way depending on the cost of the compR tires you like in either size. I will get 18x9 since, like Zstrer said above tires are moving towards that size and its not uncommon to so find 18" CompR tires cheaper than 17" tires. See Dunlop Super Sport Race tires at tire rack for examples of this (I have four of them -265/35/18- in my garage awaiting track wheels). Also, if your wheels are the same weight I believe performance sacrifices to be minimal to non existant depending on your skill level.