I wouldn't say so. Spells of the first three levels don't require much skill for either type of magic. The main thing is to have the school leveled up to the expert level. And Darkness has a really good perk – Fallen Knight. It's also great to combine mass slow with speed magic. But I think the choice primarily depends on the opponent. For example, weakening isn't as critical against a Necromancer, but it's one of the most terrifying spells against a Demon, and so on.
But you're probably right about Demons; with the Fallen Knight perk, Darkness would be a good choice against them, although this needs to be tested in practice; I haven't played that way yet.