I will go with Fitz. I see him either stopping John L late or it might go the distance. But I lean towards Fitz by a stoppage late.
As a massive Fitz fan I surprize myself by leaning towards Sullivan in a great match-up. Fitz could be hurted by lighter punchers than John L, Corbett for example and he just managed with his amazing recuperative powers to survive Maher and Choynsky. Sully was probably faster and stronger than these two, as far as we can tell, and might take Bob out. Of course Fitz might ko the bigger man but while Sullivan was sometimes dropped, I never read of him been stunned.