If they're only different by a constant factor, then both have the same interesting properties and neither is much more difficult to calculate than the other -- at least for sinusoids.
If something is proportional to one, it's naturally proportional to the other.
Almost anyone on this website could answer better than me for this, was always weak in math, but I believe they differ by a constant factor for a sine, but for a more complex waveform they will not (well, the amount they vary by would be different for each waveform).
It's true that the factor between RMS vs peak-to-peak is different for e.g. a sine vs a sawtooth wave, but for other waveforms its still a constant (just a different one), and for this engine it should be just about a sine wave anyway.
If something is proportional to one, it's naturally proportional to the other.