Hm.. It is hard to explain. But intuitively, marude sounds like "truely like" or "truely as if", whereas mitai is just "like".
彼はまるでアホのように飲んでいる。
kare wa marude aho no you ni nondeiru.
He is truely drinking like a fool.
彼は優しい人みたい。
kare wa yasasii hito mitai.
He seems like a nice man.
They're no synonyms though, as they can also be used in a combination:
彼はまるでお父さんみたい。
kare wa marude otousan mitai.
He looks exactly (or truely) like (my) father.
Marude can also be in the following way:
彼の話はまるでたわごとだ。
kare no hanashi wa marude tawagoto da.
His story is utter nonsense.
As you can see marude is always in the beginning of a sentence where as mitai is at the end. To summarize/simplify it, I'd say mitai is the simple English "like", and marude can be used, in combination or solo, to put emphasis on the likeness.