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I would say mudstone. Since sandstone is mainly quartz (60-100 percent) which is very hard and resistant (hardness of quartz=7/10) and they are chemically stable on earth's surface. So that aspect of sandstone alone makes them harder to erode. In addition, the grains in sandstone are alot bigger than grains of mudstone, which is less than 1/16mm, so sandstones would require a high energy system to erode compared to mudstones.

Another thing to keep in mind is that sometimes, muds and clays tend to be very difficult to erode because of the electrostatic charges on them. When it is mixed with water, those charges activate and it becomes very difficult to seperate them for any eroding agent(ie wind/water).

Also, mudstones are very tightly compacted together because their grains are too small and they tend to have very low porosity and permeability.