If you do deal with devils as monsters, remember they're lawful. They have to obey some sort of code. They keep their word and probably don't lie, but are good at twisting things. Really good at twisting things. On the other hand, many of them have to obey really weird laws; some Chinese fiends you can defeat by throwing rice at them, wherupon they have to count every grain. While not all laws are that weird, look at a vampire for a myth that's really, really weird if you think about it, and work from there.
Devils (may) have a kind of honour. They will surrender if you're about to beat them, and they may well allow you to escape or survive if you are about to be beaten. (Although survival as a devil's slave or servant isn't always pleasant.) However, they don't tend to enter combat all that often, perhaps because they're not allowed to overtly interfere on the Prime Plane without an invitation (ie. summoning), and they stick to that law, preferring to communicate, manipulate and corrupt from a distance, as detailed beforehand.
Overall, this treatment of devils reminds me greatly of old fairy stories about devils, who are usually quite nice except for the whole slavery and inhuman destructive power thing.
Demons, on the other hand, are just plain nasty. Their nature is lawless, and so they're free to cause whatever havoc they want. And they really like havoc. However, they are chaotic. This sort of passionate approach to evil is, in a way, a disadvantage, because demons are so evil they're perverse. If a demon lord wants to lure a party of adventurers into the Crypt Of Unspeakable Doom to set it free, it will still try to kill them on the way in because that's what it does. Demons are almost self-destructive; they're quite definitely insane by human standards.
Overall, demons are typical of more modern concepts such as Lovecraft or Barker-esque outsiders, beings that are truly nasty, both in manner and visage, and don't obey our laws. They're also more powerful in melee than devils, if you compare the ranks.
Of course, there's nothing to stop you creating an amalgam race of fiends that are both senselessly destructive and affected by strict laws... it's just less easy to do with alignments.