Goku would Crush Superman in a duel....

The tails can grow back only as a child or by artificial means in an adult.

Goku's tail grew back twice as a child but once adult apparently the tail doesn't grow back. Vegeta's tail is never grown back until the GT part of the story where it is artificially replaced by Bebi Bulma. Trunks and Goten's tails don't grow back after being removed at birth. I don't see why not since it regrew on Gohan and Goku as children. But oh well..