I don't see any difference, you can use them both equally.
Factology says that 'guy - guys' is widespread than 'fella-fellas'.
November 21, 2014
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"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).
November 21, 2014
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If speaking slang they mean the same thing. I would think 'guy' is more popular to use than 'fella'. 'Fella' is slang for 'fellow'.
November 21, 2014
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