The largest European folding bike manufacturer only builds 16" folding bikes. Brompton of England only builds 16" folders.
The largest Asian producer of folding bikes mainly builds 20" folders. Dahon builds nearly all sizes of folders but they build several different models of 20" folders.
The biggest advantage to 16" folders is they can often be built to be about 10% smaller than 20" folders. The Brompton is an excellent design.
The biggest advantage to 20" wheeled folders is the ability to use a wide ratio rear derailleur shifting system which cannot be used on 16" wheels.