MadMax has a good point. Rims will also get more expensive probably (depending on style and all that). Cost/aesthetics aside, 18s should be a bit nicer feeling.
Maybe someone can answer this as well: Does the actual size of the outer rubber tire diameter increase at all from 18-19 or is it just the rim encroaches a bit more on outer edge of a 19 than it does the 18? I thought diameter was the same but the rim size changed (slightly less wall on 19 over 18).
Also, As small of a change as this is just keep in mind that the car has 1 size by stock, if you mess with that at all you'll effect the speedometer and even possibly torque at the wheels. The actual differences will probably be so small that it would probably only be off by a few MPH (if that) nearing 100MPH, but just a thought.