If you take two samples of the same dirt and add enough water to one sample to make it mud, which sample will be most abrasive to metal: the mud sample or the original sample?

The mud sample will be more abrasive than the initial dirt sample. This is fairly counter-intuitive, but there are several factors to take into consideration when dealing with the abrasive properties of various media.
The dirt sample will act as a straight-forward, physically-abrasive substance: the particulate matter will abrade and corrode the surface of a metal through friction and physical contact.
A mud (or slurry), however, has several other properties beneficial in abrasion. The dissolution of the dirt particles will make them slightly more chemically reactive, which will contribute to the corrosive properties of the mixture and help to abrade or polish the metal surface. Additionally, the water solution will break the dirt into smaller pieces—which are inherently more abrasive—and the slurry will have a higher density (meaning more mass to abrade the metal surface).