Q: What do I give up by running this 20-inch instead of a taller 19-inch tire?

Mostly ride comfort and some pothole protection. The short 35-series sidewall on the 235/35R20 flexes less under cornering load, which sharpens turn-in and fills the wheel arch for a more aggressive stance, but it also passes more of the road into the cabin and leaves the rim more exposed on broken pavement. If you want looks and steering precision and your roads are reasonable, the 20 is the pick. Rougher streets favor the 19.