I'm hoping someone on this forum can answer your question more effectively, as I have wondered the same.
Yes, medical grade nitrous is PURE nitrous oxide... the 'Nytrous' that is recommend for use in our particular application has a sulfur dioxide added to it to prevent people from inhaling it. If you DO inhale Nytrous it is supposed to make you sick, not giggly, maybe both... never tried it.
As for purity relative to contaminants that would clog a solenoid for example... run a nitrous filter.
I've been told that there is NO difference in performance gains between the two.