Individual skills, or skill categories? Options

[Eiyu]

Just curious. In a game that offers customisation in learning and mastering skills - which do you prefer?

An example of categories; Black Magic contains every offensive magic spell. A character's potency with offensive spells is based on his/her black magic skill level, and s/he'd end up learning every spell in the category.

An example of individual skills; the more you use Fire or allocate points into it, etc, the better at casting Fire you'll get. Learning Fire doesn't mean you'll ever learn Water, nor will it increase your potency at casting Water.

Trying to figure out what the best course of action would be for my customisation. At the moment I seem to be going with categories but I'd like more feedback. Many thanks.

[Eiyu]

That makes sense, yeah. I have 3 characters and want to give them a choice of 6 classes (for now). Just wanna make sure it's not too restrictive.