Both mammoths and mastodons are pretty large groups of species. But the record for height goes to the Mammoth Mammuthus sungari who could get to be 17 ft. tall. Mammoths are more lightly built than Mastodons who are built like linebacker vs a runner. http://www.paleodirect.com/images/mammasto1.jpg
Despite being shorter, mastodons weighed as much as their taller cousins... and were likely much less friendly. We actually have a fossil from two male mastodons killing each other in battle.