Sheldon Brown's page on tire sizes says that 20" tires in decimal inch widths are ETRTO 406 mm, whereas 20" in fractional inch widths are (with some exceptions) ETRTO 451 mm.

Can't speak to 406s but I've been using Schwalbe Duranos and/or Panaracer Minits Lites in 451s and have been happy with both (I buy whichever is cheaper when I'm shopping for tires).