If I recall right, they producer more co2 than methane by a factor of 2. A better measurement is pollution, but then we would need to attribute all the particles which makes it more complex to compare the burning of fossil fuels with gas produce by fermentation.

Not to say that I am not a fan of less selective counting. I in big favor of counting footprint based on a person complete effect on the environment rather than specific choices. If we counted everything from purchases of items, clothes, travel, commuting, diet, and so on, I am convinced that the end result is more enlightening than just looking one diet item vs an other.