A shorter sidewall (19”) will provide less flex which makes steering feel more precise. A taller sidewall (18”) provides more cushioning which acts to absorb impacts better making for a more comfortable ride.
My Roadster came with 19's and I found the ride was seriously harsh even with Mag Ride turned off. Swapped out to 17's with a new set of Conti tires and it made a world of difference. It's a proper touring Roadster now.
Yeah my car (TTS coupe) came with 18" wheels--everything was pretty much stock when I bought it a couple years ago. I was disappointed in that at first because in the US the S came standard with 19" wheels, while in Canada it was an option...an option this car wasn't equipped with which was strange given it had pretty much everything else.
Anyway in retrospect...call me old...but I'm glad it had the 18" for the softer ride. Instead of replacing the wheels with aftermarket 19s (I had thought about that at first) I just had the [badly] rashed stock 18s refinished and am quite happy with 'em.
19" is def. gonna look better but living with it daily, you might not like the ride.
I have 19" with stiffer suspension on mine and drive it around London. It's firm but not uncomfortable. I can't say I really noticed any difference when switching from my 18"s. Engine size isn't important