If the reason you're going for 19's is "extra ground clearance".... then you're going for something with the wrong tyres!
The rolling radius should stay the same, which means the overall tyre height stays exactly the same: so the ride height stays the same.
19's are very pretty if you're doing lots of show 'n shine or want to impress the kids in the local McDonalds car park, but not for a daily driver that is being used on anything other than German quality motorways.
Things to bear in mind: pot holes and kerbs. Those two are enough to mean I'd never go for 19's - Audi NEVER fitted 19's to a B6 and there a reason for it.
I've gone for oversized alloys before and never again - I couldn't park outside most of my friends' houses (against high kerbs or one-wheel up on the kerb) and it felt like the rims were hitting the ground on some of the ridges in the M25, let alone the one time I split a tyre on a pot hole.
I'm very happy with the S4 18's and wouldn't change them over on a free swap for any set of 19's (and a free set of the tyres I'd invariably split with the amount of country A and B-road driving I do).
But then I am risk averse: I prefer to spend money on good tyres and then protect them - 18's for summer and then I use 16's in winter with proper winter tyres, which saves the extra wear you get on non-winter tyres*.
*Below 7deg C, non-winter tyres wear out 40% faster than winter tyres - but I only used those because theyre the OEM sport alloys, I'd recommend 17's as a decent compromise of being smart and pot-hole proof for winter tyres, but only if u have a set of 18's for your summer tyres