I'm a bit unclear about a couple of things regarding souls and enchanting.

I've gleaned that human souls are 'black souls', while all the creatures have 'white' souls. But I just killed two soul-trapped bandits (including one bandit chief) and now the only two black soul gems I have have grand souls in them? The wiki says bandits have grand souls, but also says that all NPCs have black souls. Does it mean only named NPCs have black souls?

How exactly does the level of soul affect enchantment strength in weapons? In apparel I see that the effect varies, but with weapons it only seems to affect how fully charged the enchanted weapon is? It also doesn't seem to make much difference (like, 10%) between, say, grand and petty souls. Am I missing something?

In general, what is the way to go for enchantments? More charges, more power, or somewhere in between?

Humans have Grand Souls, but they can only be stored in Black Soul Gems.

As for enchants. I personally go with more charges less power.

For example I mainly use Fiery Soul Trap and Paralyze

FST will always deal 10 fire damage. I just put it to 1 second. Same with Paralyze.

By a black soul they mean it can only be captured in a black soul gem, the strength of the soul (Great, grand lesser) varies on how strong the enemy is

At the first part all black souls are grand souls.

At the second bit, the size of the soul changes how strong the enchantment can be. On weapons this is number of charges , while on armor it is what % it is. 10% is actually a large difference while enchanting. 10% more weapon damage can be anywhere from 1 damage to about 60. It just depends on alot of other factors, but it is a huge difference.

A black soul gem is the same as a grand soul gem, But the black souls go in black soul gems and are called grand souls, white souls go in the others.

What they said.

I see! That clears it up, thanks.

Yeah, but it doesn't translate to more damage, does it? Just how full the charge bar is.

Another question: for the Soul Siphon perk, do draugrs count? They're quite a common enemy with all the crypts around, otherwise the perk doesn't seem worth it.

This is the how I apply soul gems...

Petty and Lesser - Save for leveling your Enchanting skill.

Common and Greater - Spend to recharge your weapons and Staves.

Grand - Use to enchant gear you use.

Yes.

On some enchatments it does give more damage but I don't know which or how much off the top of my head.

As for soul siphon, I have no idea as I've never taken that perk.

Except in case of black souls, they are grand regardless of the NPCs level.