Well if you go by what the harman manual says the white. For one thing it won't create what they refer to as clinkers, or as many. That's one comment I get from many customers when they switch to me from some other dealers coming from some areas where they are getting the red ash. I've seen clinkers the size of softballs in some of their ashes.
A clinker is created by the non-burnable parts of the coal, essentially it's rock melted together. This can be quite a hassle if there is a lot of them. The white will have some but relativeley few, small, and they easily fall apart when you rake the stove if you're using a hand fired stove.