Coal depends on the 50% of the wood, 25% of the kiln, and 25% of the fuel. Since you will use boards is used in the wood and as fuel, the variable is kiln which will influence the coal quality.
If kiln q > board q, make coal and use those as fuel.
If kiln q < board q, use board as fuel.
Coal is generally better because you can spiral its quality if you have decent clay, that's all.
Actually if the person has a strong saw made of decent branch and at least bear quality bone, the board can easily be higher than the coal and high quality bone saws can be sold from many different merchants in this world. If you didn't buy a bone saw from someone, the biggest influence is the tree quality as it affects the branch for the bone saw and the board you make from it.
However, since you can't use board for smelter and coal is required for steel, you will need coal regardless.