when i bought my first folder 10 years ago, i test rode a variety of 20" and 16" wheel folders.

i much preferred the ride quality of the 20" wheel for chicago's FAR less than perfect streets.

and i wasn't looking for absolute maximum compactibility, so i went with a 20" wheel folder.

it seemed like the best balance between ride quality and folded size for what i was looking for.

but if you're planning to take your folder on a bus/train for everyday multi-modal commuting, then a smaller wheel is probably the better way to go.