The 19" wheels will be more comfortable because they have more sidewall and they will have much longer life--perhaps double or more.
The 21" wheels will be faster off the line, primarily because of the stickier tread compound.
The shorter-wider contact patch of the 21" tires will deliver more g-force.
Both will have the same contact patch area if run at the same pressure because contact patch area mostly depends upon the air pressure in the tires (and a very small amount on sidewall stiffness).
The 19" tires will most likely be better in the rain because the tread width is narrower. In this case I say most likely because sometimes tread compound more than makes up for the increased width.
The 21" tires will become very hard, and lose traction, when the temperature drops.