For an isobaric process, we wish to use the
Enthalpy, for the pressure appears in the
differential there. This choice allows us to carefully account for the
work done on the system and that done on the environment in a process.
For a process that is both isobaric and isothermal, it makes sense to
look at the Gibbs Free Energy.
Therefore, while solving problems, look for what is being held constant
so that you can make an appropriate choice of energy.