as far as tires go there are tons of options
options as far as sizes, widths, traction, temperature, treadware...
imo the 215/45 17 that come stock are small...too thin of a sidewall...
honestly 17s might seem a bit small bit if wanting to stay with the 17s then ur options from there would be 225/45 or a 235/45 giving you a wider tire which in turn puts more meat on the ground and effectively raises the sidewall of the tire up just a tad eventhough they are still /45s...this will give you a smoother ride and less jarring....rule of thumb is first number is ur width across the tread in millimeters...the second number is ur sidewall height in a percentage of the width...so a 225/45 breaks down like this...225mm wide and 45% of 225 is your effective sidewall height so if u take the sidewall and lay it over the treads effectively it would be 45% of your tread...thats a break down of tire sizing hoping it would give you a lil better understanding....
now onto 18's...
same thing applies with 18s ur options would be a 225/40 18 or a 235/40 18 which will be a bigger tire...therefore with a shortend explination i would go with the 18's even though they are bigger they are lighter setup per wheel and tire for whatever reason a wheel/tire combo in a 17 is heavier than a combo in an 18...hope this helps ur wheel and tire section