Using the standard image-to-heightmap function, Rhino, MeshCAM, and MeshCAM Art are identical. MeshCAM does have a nice smoothing function that helps a lot for some images but it's still a straight heightmap conversion.
Art adds additional functions that let you flatten the heightmap out and then rebuild it as a series of shapes from different regions of the image. In MeshCAM Art, these regions are selected by color. The help file has an Art Quickstart to show this functionality in detail.
Robert