I wasn't much of a Man-Thing reader when I was young. I preferred Swamp Thing over at the Distinguished Competition. I think it came down to the art. No disrespect meant to Mike Ploog, but I liked the art of Berni Wrightson and Nestor Redondo more. Even the last issue done by Ernie Chan (with inks by Fred Carrillo) had much more appeal.
Also, one thing I liked better about Swamp Thing is that he retained Alec Holland's mind, while Man-Thing's Ted Sallis persona was buried deep within the mind of the muck-encrusted monster.