Which is more _wasteful_? I would argue Bitcoin, and dangerously so, because its upper bound of potential energy usage is infinite.
Coal, on the other hand, is not wasted (burning coal without using the heat would be stupidity).
Even when coal is phased out, Bitcoin may still be there, causing unnecessary strain on the infrastructure.
Bitcoin will find the cheapest energy and set it on fire, if the price is right. And the heat produced by the equipment also wasted as byproduct due to convenience/commercial factors.
I can imagine it already contributes to energy poverty, alongside other issues, in developing nations.
bitcoin vs coal
One of them is responsible for a significant percentage of global carbon emissions and the other is not.
I don't favor bans, but at least that thought experiment should indicate which one to go after in some way (e.g. taxes).