20GB will be MORE than enough... I have several saved games, downloaded demos and trailers and have only used around 3-4GB. I imagine the game saves will be under 64MB simply because thats the smallest memory card that the 360 uses.
As mentioned though, Sony is no longer producing 20GB versions for North America... there may still be some out there in stores but you better get cracking.
As for the differences... the 60 has wifi and flash memory readers which the 20 doesn't but they can be added on later with off the shelf devices.