From what I know the 20" inch is only in a rib pattern with tread depth of 3.5 mm,,21" gives one a better choice of manufacture and tread patterns I've been going for the Avon Distanzia with much more tread depth on a WM 3 rim or wider the max recommended being 2.5" much more support better ride and more readily available, no other choice if you want value for money.
bananaman