It will really come down to your tastes!
20's fill in our enormous wheel wells... trust me I have 20 x 12 +0 on 305 series tires.
19's are great as they are typically a pound or two lighter.
Other than looks and weight one is not necessarily better than the other. I will say 20's will be a slightly stiffer ride quality and you will have more feedback in your steering wheel (I like that aspect of the 20's)
However I have found a compromise between the two sizes. I went with Strasse Forged 20" step lip giving me wheels that are super light while giving me the looks of 20's:
The nice thing about 3 piece wheels is that you can do any combination you want!!! You get to design your own wheel! We can help you with offsets, tires and even color choices!