That depends on how big a percentage of the cost fuel is, and what government fossil fuel policy looks like. I find it far more likely it will be the other way

That depends on how big a percentage of the cost fuel is, and what government fossil fuel policy looks like. I find it far more likely it will be the other way around: current tests are using natural gas, and I'm working under the assumption that SpaceX will switch to synthetic methane only when they have very high margins on the Starship business or fossil fuel taxes/permits start to bite. (Happily for the environment, this is still a drop in the bucket.)