sexy 996! Go for 20s, you will compromise a little ride quality, but your car will look so damn sexy that you wont even care

 

The ride quality is more affected by the profile of the tire than the diameter of the wheel. To keep the overall diameter the same as your original tire/wheel combination a lower profile tire has to be used on a 19 inch wheel.
For example, 18 inch wheels on a Buick will have a softer ride than 18 inch wheels on an E46 because the Buick will have a higher profile tire on their 18 inch wheel. The profile on the Buick tire is 50 versus the 35/40 on an E46.
So, although the ride on 19 inch tires might feel fine on your Porsche, there will probably be a difference in ride quality for your 530xit when compared to the ride with the original wheels.