The main benefit is that 18 inch wheels look cooler to some people. Also, depending on the wheel and tyre choice, you may lower the unsprung weight slightly by using 18 inch wheels, which theoretically improves acceleration response.
The main drawbacks are that the tyre sidewalls will be lower. This makes the ride quality harsher and the rims more vulnerable to pothole and kerb damage.
I have no advice on a good wheel to choose, unless you go for the standard 18 inch GT "jetfin" rim. In terms of tyres, I'd recommend GoodYear Eagle F1 Asymmetric.