I have looked them both up at dictionary.com and they are synonyms; however, it seems that Marquess is more "british". My novel is set in 19th century London, England- does it matter which one I use?
I am just curious to know which of the two your chose for your book? I used to go to a pub called the Marquis of Granby and that was in a little village in England in the late 1960s. May be they couldn't fit the longer spelling on to the sign outside the pub!