I didn't vote as I think they are both nice. I'm going to try and fit the S 21" wheels on my 3, assuming the offsets aren't way off. Outer diameter might be too big though and throw off speedometer too much.
The silver alpha car had 20" originally. The 19" wheels we see now are of the same design. So either they rethought the 20" and went with 19" instead or 20" of the same design will be available later on the performance version.
It appears to me the OP/vendor is simply asking which wheel do you think looks better. My guess is that will be the wheel they try to produce/manufacture/stock for the Model 3 as an aftermarket alternative. So the size doesnt matter for the sake of this poll.