Renewables to methane probably makes more sense. Methane is cheap and easy to produce. Using it skips the additional step of methane to methanol conversion. There are already plenty of methane powered cars on the road in Europe and a few on the road in the US. Renewable production of methane is already being done at scale by BioSNG in Sweden.

This claims that it is not yet competitive with untaxed natural gas from fossil fuels though:

http://www.biosng.com/fileadmin/biosng/user/documents/report...

Presumably, the taxes in Sweden are sufficient to make it feasible.

Methane still forces you to run on the Otto cycle or a variant thereof, not the more efficient Diesel cycle, so unless engine makers get HCCI right, renewables-to-diesel would still be a better choice.

The only thing that matters for adoption of renewables in transportation is $/mile. The efficiency of the heat engines is irrelevant.

Natural gas is cheaper to make in a renewable manner than diesel (Europe is already mass producing natural gas from biomass) and has higher energy densities in J/g, which makes it suitable for transportation. The lower combustion temperatures do lower efficiency, but they also lower NOx emissions, which are a difficult problem for diesel.

As far as aviation is concerned, natural gas could be more efficient not only due to the lower weight, but also due to the fuel being "precooled" in its liquid state:

https://en.m.wikipedia.org/wiki/Precooled_jet_engine

Butanol!

It has better energy density than ethanol and absorbs less water than ethanol and is not as good a solvent as ethanol (so engine parts last longer).