Charcoal is made by burning wood in a reducing (vacuum) atmosphere. It is usually made from hardwood with low pitch content.
Activated charcoal is made from coconut shells and bamboo woods because it has a porous nature to begin with. the wood is burnt in a reducing atmosphere and then injected with oxygen at the end which burns out the insides of the wood pourosity. Regular charcoal does not have this good internal pourosity. The introduction of oxygen under vacuum draws the oxidizer deep inside the wood allowing a good burn throughout.
Good activated charcoal will sizzle when introduced to water. The action of the water inbibing all the porosity will force out all the air and make a cool sound. Not only that, it smokes!
Now, understanding what the difference is what are the benifits to each in the case of bonsai soil?
Charcoal does three things to soil.
1. It acidifies the soil
2. It adds benificial humic acid to the soil
3. It provides a place for bacteria to grow to aid in the breakdown of organic fertlizer into compounds usable to the plants roots.
The addition of wood either as wood or charcoal will acidify soil. The good thing about charcoal is that it does it without robbing nitrogen during its decomposition process.
The number one place to get superior humic acid is from leonardite, a type of soft coal. Basicly a type of more decomposed charcoal. Coal being just highly compressed charcoal.
Porosity is paramount is providing a home for benificial bacteria in the soil used in the breakdown of fertilizer.
Adding charcoal to the top of the soil is benificial for the humic acid it may release due to watering, but not the production of nitrifying bacteria within the charcoal. Mixing it in the soil will provide for that.
If adding wood to your mix, and charcoal will break down depending on repotting schedules, then adding humic acid to a soil mix will remove the need for the charcoal in the first place. The addition of humic acid as a liquid or in fertlizer begins to acidify the soil immediately. The addition of pumice in the soil provides the habitat for the nitrifying bacteria to breakdown fertlizer.
Or....just do it more simply by making a soil mix of 1/3 akadama, 1/3 lava and 1/3 pumice. Use organic fertilizer cakes to jump start the humic acid in the fertilizer and you have done the same thing without the addition of another hyped Japanese soil component.