how much does 18" rims effect performance?

I want to get rims but i need help making the decision of 17 or 18 inch. I have heard the 18"s can slow it down some and i was just wondering how much?

It can. It depends largely on the weight of the wheel. Without that info you can't say how much acceleration is gonna get killed.

And its basically looks (18s) vs. a slight performance gain and practicailty and price (17s) ....

Let me put it this way;

Lightweight 18's are still slightly worse than moderately weighted 17's

Its all about rotational mass.

Yeah, because the weight is farther from the center of the wheel in an 18 than it is on a 17.