The difference between generating a DEM directly from the Dense Cloud or the Mesh is the way Photoscan interpolates the data from a point cloud to a solid model.
When you choose to build a DEM based on the Dense Cloud, Photoscan will generate a DEM on the specified resolution, build from the the dense points directly with an "Inverse Distance Weighting" (IDW) interpolation.
The DEM resolution that is specified by Photoscan is directly related to the Quality mode that was chosen when building the Dense Cloud. Where a higher quality level means a higher density 'Dense Point Cloud' and thus a higher resolution that's available when building a DEM (without upsampling the data).
When you choose Mesh, the points get interpolated as a "Triangulated Irregular Network" (TIN) where it linearly interpolates the Dense Points as small triangles. The higher the specified resolution is, the smaller the triangles are.
Both are fine interpolation methods, but for surveying purposes I'd advice to use IDW. This method also usually generates less noise around building and structural edges.
An Arbitrary Mesh is when Photoscan tries to interpolates all the dense points through a TIN method. Suitable for when you have a Dense Cloud of a statue or an object.
When you're dealing with stockpiles, you would only need a Heightfield Mesh.
This is when Photoscan produces a TIN from only one viewing angle; the one that's perpendicular to the red lined side (the underside) of the bounding box.
If you have your Point Cloud geo-referenced you can always reset the bounding box. This will automatically align the underside facing the earth for when it's not correctly placed when wanting to create a Heightfield Mesh.
For creating a model of a stockpile, you would only really need the highest Z point at every XY location, therefor a Heightfield Mesh is sufficient to produce an accurate volume if you've flown a proper nadir aerial grid over the stockpile with decent overlap.