What if Duck Anderson's Green Herb cards healed for less but you got to keep unused Green Herbs between rounds? Compared to Donnie B. you get less extra health per round but you could stockpile multiple Green Herbs for a ton of healing in a single round.
Another way to make Duck Anderson more distinct could be by making Green Herb heal for more health but you heal slowly overtime. That would give Duck Anderson the edge in prolonged long range fights but weak to close range burst DPS.