[**Spoilers For Various Comics Featuring Swamp Thing And Man-Thing!]
Marvel’s The Man-Thing and DC’s Swamp Thing share some similar qualities, besides being swamp creatures. Like most people, I thought these qualities meant they were essentially the same concept. However, I have done some reading on both and found that even in the beginning, they were only slightly similar. For example, Man-Thing first appears as Doctor Ted Sallis in Savage Tales #1 (1971), and transforms after his accident involving his secret formula. On the other hand, Swamp Thing (Doctor Alec Holland) would have two first appearances (House Of Mystery #92 in  and Swamp Thing #1 ) with essentially the same basic premise.
Yet, there are still other differences between both characters such as: Their designs are only roughly similar. While both are essentially naked swamp creatures, the designs of their faces, fingers, and shoulder areas are very dissimilar. Also the way each character’s story gets expressed is extremely different: Man-Thing is often lacking drive and mostly follows the writer’s narrative, and he is more stuck in the horror, science fiction, and fantasy genres (Essential Man-Thing Vol. 1 ). While Swamp Thing’s intellect drives him to action, yet his tales lean more toward the writer’s preferred genre (Challengers Of The Unknown #87  and Saga Of The Swamp Thing vol. 1 ).
Another big difference is the two most notable characters to come out of their respective titles: Howard The Duck and John Constantine. Howard comes from a somewhat forward thinking fantasy type story starring Man-Thing, but goes on to become a satirical lead character (Adventure Into Fear #19  and Howard The Duck #1 ). John on the other hand starts out as an homage to musician Sting, and as a way of introducing more mysticism into Swamp Thing’s mythos (Saga Of The Swamp Thing vol.4 ). Not to mention that John has become a lead character who is less homage and more an anti-hero who flits about genres much like Swamp Thing (Brightest Day Aftermath: The Search For Swamp Thing issues 1-3 ).
Finally there is the mainstream success factor. Swamp Thing has starred in two movies, a television series and a cartoon show (Respectively they are: Swamp Thing , The Return Of Swamp Thing , Swamp Thing [1990-1993], Swamp Thing [1990-1991]). Man-Thing has starred in only one movie (Man-Thing ). Yes, both have guest-starred in other things, such as Batman and Harley Quinn (2017) and Hulk: Where Monsters Dwell (2016), but their respective popularity are at vastly different levels.
In conclusion, though there are similarities to their starting points these two characters have grown in different ways from the seeds they were.