Regarding the abrasive properties of Sand vs. Volcanic Ash, if you had an electron microscope handy with samples of both available, you would be able to tell the difference in abrasiveness simply by looking at them. Fortunately, such images are also available online.
First, Sand:
Second, Volcanic Ash:
However, as previously mentioned, the abrasion on the airframe and compressor blades in the engines is a secondary consideration to the melting of ash in the combustion chamber and turbine blades in the engines.