[Berzerker]

It will be noticeable in roughness driving changing from the thicker sidewall of the 17's but the difference between 18 and 19 themselves will be minimal. The biggest factor will be the chance of damaging the rim itself from pot holes and stuff. The thinner the sidewall the more likely your going to get bent rims. If your doing a lot of commuting (and being a female driver) I would likely stay with the 18's and have that little bit extra cushioning of the extra sidewall thickness.
Also 19" rubber for whatever reason is always more expensive than 18" and 20" I guess it's a supply and demand type of thing.
One final thought on an SL500 you could easily roll on 20" rims if you were really just going for the look. I'm sure you could find some decent wheel and tire packages for 20"
Berz out.