Is it appropriate to use PSD on the analysis of a plucked guitar string?
I'd really recommend going for a short-time fourier transform such as that produced by: matplotlib.mlab.specgram. It gives you the change in the spectrum over time and can be calculated using PSD or the magnitude spectrum.

The reason I'd recommend this is that your guitar signal is not spectrally stationary; your graphs are only a snapshot at one point in time. What gives your guitar string their tonal differences is the way all the harmonics shown above change over the length of the note. 

There's a very good summary of PSD vs FFT magnitude here and here.