i always assumed that greyback is different from lupin in that sense that greyback consciously chooses to be a werewolf, and embraces his lycanthropy - he sees it as a liberation, while for lupin it's a curse. greyback also feeds on humans when the moon is not full, (bill weasley) - i take it this is why he also looks more feral than lupin does, who struggles against his werewolf nature and even locks himself in so he doesn't feed on others.
i actually like greyback - he is just the opposite of lupin. - he rejects his human nature and believes in the beauty of being feral and all. a bit of cliche maybe, but kind of cute. or maybe 'cute' is not the right word here.
nevertheless i don't think the werewolf is a separate part of lupin: bill weasley was only bitten by a not transformed greyback and develops a preference for raw meat afterwards - although he never turns.
and the heightened werewolf senses - like smell and sight - are probably not canon - but they are so useful for fan fiction writing *grin*
'wolf' seems like an endearment almost, while 'werewolf' might be more of an insult since lupin detests his affliction so much.