Well i have 18's... they were with it when i bought it.
I kinda wish i had 17's though. The tires for 18's are outrageously expensive. The tires equal or are equal to more than the rims themselves (if you get good tires).
17'' tires are cheap as balls... get 17's.
You'll actually go faster with bigger rims and bigger tires, but they'll weigh more... your top speed will be maybe 2-5mph faster... so not worth it in weight, cuz the extra lbs will hurt your accel a tad as well.
im trying to decide this myself actually....18's fill in the spaces so well but 17's are cheaper and lighter...id possibly get 17's and drop it a little more and pray to god my longtubes dont scrape on everything haha
my friend has 17s and his is lowered its scary following him and watching him go over small bumps. he almost scrapes his cats on the speed humps and if he were to ever attempt to go over a speed bump hed surely mess up his cats.
i'm running 18's. they look so much sexier than 17s!!! and the 18's with the tires are the exact same height as the 17's with the tires so its not a big deal on speed. maybe a little heavier but i never noticed the difference.