I'm sure everyone here remembers those tables in the back of the 1e DM guide (?) for randomly generating daemons/demons/devils. I would like to take an opposing approach and give each race distinctive visual styles to go along with their distinctive origins, modus operandi and places on the alignment chart.

What do you suggest would be suitable overarching visual motifs?

I personally would make devils fine-featured, human-sized and almost human in appearance, save for skin-colour (often dark red or maybe a blue or green colour depending on their chosen sphere of influence), hooves, small horns and possibly a small tail - but they would often be clothed.

Demons I would make monstrous in a vast variety of ways, usually unclothed and rarely very similar to human appearance. Just about the closest a demon gets to being human in appearance would be something like the Balrog or a demonic imp - it's got arms and legs, but it's got wings, and either a tail or a monstrous countenance, perhaps claws, weird demonic eyes, horns whatever.

The main differences here are that the demon  is unlikely to be human-sized and is likely to have wings or other animalesque appendages.