SourceDiffusion solves the diffusion equation for a scalar quantity, while SourceViscosity solves the diffusion equation for the velocity (a vector quantity obviously). In the case of a spatially uniform viscosity the two are strictly equivalent since—in the case of an incompressible fluid—the vector diffusion equation for the velocity can be split in independent scalar diffusion equations for each of the Cartesian components of the velocities. To sum up, using SourceViscosity is safe in all cases, regardless of whether the viscosity is uniform.