When simulating spectral line broadening, which convolution is preferred?
I'm not familiar with the computational packages -- I'm an experimentalist, not a theoretician. 

As an example for why both might be present, we can turn to gaseous infrared spectroscopy:
one will quickly find that the line-widths in vibration-rotation spectra depend on pressure. At lower pressures, say less than a torr, Doppler-broadening is the main mechanism. This is Gaussian in nature. While at higher pressures the broadening is instead due to collisions, which are Lorentzian. You'll find combinations of the two and so forth depending on the specifics of your case.