I don't see any difference, you can use them both equally. Factology says that 'guy - guys' is widespread than 'fella-fellas'.

"guy" is more popular unless talking about someone's boyfriend, etc. (e.g.: Stop looking at my fella. He's taken.) Also, "fellas" are all male, but "guys" can be any gender (although less often all female).

If speaking slang they mean the same thing. I would think 'guy' is more popular to use than 'fella'. 'Fella' is slang for 'fellow'.