It depends, I try to choose something that's easy to read to avoid confusion between some characters and numbers. Also some programming fonts offer symbols ligatures. I don't like them, maybe it's just an habit but, if I have to write something, I don't want to think about the meaning of a symbol into another font. I find my self comfortable with the Ubuntu fonts, these are my system & browser defaults. Being everywhere in the interface I forget about them and concentrate on the code. Color palettes and code highlighting are also my concerns, I change them depending on ambient light. But I usually prefer bright palettes.