I like the Balor, and pretty much every artwork seen to date (except for Efreets), but I agree that they're not different enough from Pit Fiends.
A color change would have been much appreciated, with red remaining tied to the big devil, but I see they considered it, since the Balor has a much darker hue.
Maybe the things that strikes me the most is seeing the Balor better armored/clothed than the Pit Fiend... Come on! The demons are the bestial ones, the devils are the sophisticated. Pit Fiend needed a cool hell-armor or hell-clothing. Balor only needed much fire and skulls. I see they were going that direction, but then why the naked Pit Fiend?
The angels are by far my biggest favorites. Wow. Maybe I'd only have preferred the elemental bottom being tied to "genies", but it just adds awesomeness to angels as well.