If that would be your choice of fish, the 20 gal tank will be big. a betta and a snail can do well in a 5 gal tank.
As what Zildjianman said, 20 gal is not really a small tank. if you still want this size, what type of fish do you really like? community or single-specie tank? aggressive or peaceful? fish colors?
About cycling a tank...The reason i know why we do fishless cycle (besides from what is said above) is that the cycling time is short and once youve cycled enough, you could put the fish you want at the same time.
