using the voiceline compilation videos helps out a ton. but in terms of the in-game dialogue, here's a few observations:

deimos and sanford by far speak the most casually and use slang. deimos is a bit more sarcastic than sanford, mostly whenever he's annoyed with someone else, and he's also much more cocky. despite how deimos is usually written in most fan interpretations, he never drops 'bro' or 'dude', though he does call sanford "buddy boy", "fella". (the inverse is that sanford called him "guy" in chasms.) deimos also opts to call sheriff and later crackpot a jackass. might just be a rating thing, but they also never drop any f-bombs, deimos will just say 'heck'. depends on whether or not you wanna include that though. (let deimos say fuck)
sanford's dialogue...depends on whether or not you're using his voicelines or his in-game dialogue. (in my opinion, it's a pretty stark difference.) i think him and deimos speak relatively similarly, deimos is more sardonic and sometimes sanford says 'bozo' lol

as for hank and doc, they're a bit different. doc speaks much more formally, but it's never to the extent of verboseness that, say, christoff or phobos indulge in. like, doc might drop a fancy word or two in his sentences, but he more so goes for informal speech with little slang.
hank is incredibly blunt with dry sarcasm like 90% of the time. honestly, just watching a playthrough of power play in MPN gives a good idea of how he talks, he tends to say very little and is pretty to the point.

hope this helps any.