So i have a question.. I am all new to enchanting, never really used it in any of the TES games..

Now i made a mage, and was thinking of having enchanting on her.. So i have a question..

Which soul gem is stronger? The grand one or the black one..

And what does it matter which gem i use? Number of charges? "Stronger" enchant on armor parts?

They are the same, but Black Soul Gems can capture the souls of humanoid NPCs like Orcs, Nords, Falmer, etc.

So they can be much more useful when clearing out most caves because you deal with a lot of bandits and other humanoid enemies.

Both are the same, main benefit of black gems is that you can fill them with bandits, use mammoths for grand.