I'm not that experienced, especially not with character art, but I do have a feeling of what's good and what isn't. I think Vincent nailed it. It really is a question of balance. What I would do is put as much detail into the shape of the face with geometry without going too crazy.
I don't think there is one solid answer. It really is about balance. If the detail requires the geometry, you should model it. In your case I think you should model (or sculpt since it's more natural). In other cases, it might be better to paint them. Thankfully, if you were to sculpt your character, you could always decide how much detail you want to put in when you're retopologizing, and then that might decide whether you'll need to paint detail or not.
One hand washes the other.