We recommend the SWG be at least 1.5x the size of the pool. The larger the cell, the less hard it has to work to make x amount of chlorine, which means the larger cell will last longer before needing replaced.
The cost difference between a 20 and 40K unit is not all that much, so I would say go for the 40 if you can spring for the price difference.
Hayward is likely the most popular brand. Its easy to operate and has the less expensive replacement cells and easy for anyone to replace a board if its needed.
Pentair and Jandy area also very good brands. I have a pentair and I like it, but the replacement cells are very expensive. Someday, I'll have to pay out the nose for a replacement cell. Pentair has all of the electronics built into the cell, which is why its so expensive.
I dont have any personal experiece with a Jandy....