Most scouts seem to envision Ervin as a tweener. The consensus is that he's either a mediocre defensive CF with a good bat, or a good defensive corner OF with a mediocre bat. Billy is viewed as a potential game-changer, with the physical tools to be special with the glove in CF and to be the kind of weapon at the leadoff spot not seen since the 1980s.
Billy clearly is the bigger 'boom or bust' guy IMO. If he hits his ceiling, he could completely change the landscape of the game, but if he hits his floor, he'll never be more than a speedy bench OF. Ervin has a much smaller space between his ceiling and his floor, and his floor is likely to be a lot more useful than Hamilton's.