Biomass: Digestor/Reactor vs Gasifier Efficiency

When using biomass for energy, two popular options are reactors and gasifiers. The first uses anaerobic micro organisms to convert the biomass to fertilizer and a methane rich gas mixture. The second uses heat to pyrolyze the biomass into ash and a carbon monoxide and hyrdogen rich gas mixture.

In terms of the overall potential energy produced by their resulting gas mixtures, is there an efficiency difference between the two?

For example, if you left one unit of wood in a digestor/reactor and let it sit until it was completely consumed by the system would the resulting amount of methane rich gas have more potential energy than if we ran that same unit of wood through a gasifier and captured the resulting amount of carbon monoxide and hyrdogen rich gas.

Great question. At the level of basic chemistry, what I would do is compare the energetic content of what is left behind by either of the processes. The sludge remaining after the methanogens have done their duty in a biogas reactor still has some carbon in it, so unless you're drying and burning it, that's left on the table.

There also is the heat loss, not sure which is more-the fast hot or low and slow.  

I think there is a LOT of variables in efficiency based on the specific implementations.

I think this is the wrong comparison. Wood is high in lignin and it will not digest or actually it will just take forever. If you want gas out of wood there is only one choice and that is pyrolysis.