i'd get 18's. oh and good choice on rims, they are very sturdy and can take a good beating.
wouldnt 17's be better for racing?
in my opinion there really isn't a difference in speed. try it for yourself. i just had 18's and switched back to my stock rims until my tires get here. if anything it felt like my 18's were faster. i always heard that it would be slower, but if you think about it, it's not that big of a difference. i love my 18's.