Neither. Propanediol is just a protic solvent and MSM a solid which, due its close relation to DMSO, is often called a penetration enhancer without reliable citation. MSM may have other effects on skin penetration as you can read HERE (not saying that this is a reliable source nor that the results may be extrapolated).
I don't think either are true penetration enhancers the way Dimethyl Isosorbide is, for example. It's like when people say glycerin is a penetration enhancer.
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