It's a bit more complicated than that.
Both tools have sketching functions, solid body creation etc. DM uses a parametric tree structure where as SCDM (SpaceClaim) doesn't. Initially this made DM better for creating and altering geometry that's created just in the Ansys tools. However, where CAD is imported it's less useful and can make clean up more difficult: SCDM is better for that, and tends to be significantly faster for large models.You're unlikely to see the speed effect on Student as the models are smaller (ie less faces).
After several releases many commercial users use either tool for either purpose depending on what is available and preference. SCDM gets more development effort and has better links into Fluent Meshing (I cover Fluent as part of the technical team) so is probably the better tool to learn at present. But also remember that Ansys is CAD agnostic so more often than not are using models created in one of the more commonly used CAD tools. If you weren't aware many large companies have a large CAD team with a smaller simulation team taking models from the CAD Engineers.