Light or Dark magic.

Oh my god, I'm scared of you!))))))))))))))) Are you also using Chaos for the Barbarians?)))))
Well, in any case, Light Magic will be better because it is much less tied to Sorcery than Dark Magic. The duration of the effect and the number of units resurrected depend on Sorcery, but Dark Magic is more strongly tied to Sorcery, and since we have problems with it...
Although, I can provide support with Light Magic through units that use Sorcery (monks, angels), but only a hero can cast Dark Magic, so the answer is not so clear-cut, but I still prefer Light Magic.

I wouldn't say so. Spells of the first three levels in both types of magic do not require a lot of Sorcery. The main thing is to have the school leveled up to Expert. And in Dark Magic, there is a very good perk - Fallen Knight. It's also great to combine mass slow with haste. But in my opinion, the choice primarily depends on the opponent. For example, weakening is not so critical for a Necromancer, but for a Demon, it is one of the most terrible spells, etc.

2 Cr@zy Wolf
You're really doing something strange with Chaos. What Chaos for a Knight? He has no Knowledge, no Spellpower, and no spells in the guild. I advise you to reconsider your views.