Do investors in high tax brackets or those in low tax brackets benefit more from tax-exempt securities? Why?

Do investors in high tax brackets or those in low tax brackets benefit more from tax-exempt securities? Why? Do municipal bonds or corporate bonds offer a higher before-tax yield at a given point in time? Why? Which has the higher after-tax yield? If taxes did not exist, would Treasury bonds offer a higher or lower yield than municipal bonds with the same maturity? Why?