We propose that minimum incremental cost per unit of greenhouse gas (GHG) reduction, in essence the carbon credit required to economically sustain a renewable energy plant, is the most appropriate social criterion for choosing from a myriad of alternatives.  The application of this criterion is illustrated for four processing alternatives for straw/corn stover: production of power by direction combustion and biomass integrated gasification and combined cycle (BIGCC), and production of transportation fuel via lignocellulosic ethanol and Fischer Tropsch (FT) syndiesel.  Ethanol requires a lower carbon credit than FT, and direct combustion a lower credit than BIGCC.  For comparing processes that make a different form of end use energy, in this study ethanol vs. electrical power via direct combustion, the lowest carbon credit depends on the relative values of the two energy forms.  When power is $70 MWh^-1, ethanol production has a lower required carbon credit at oil prices greater than $80 bbl^-1.