The main attraction that made me buy Plasticity is the fabulous filleting of the Parasolid kernel. But, after having created one project with Plasticity, I concluded that I still like MoI 3D more, particularly because of its curve tools.
Having said this, I do think that SDF modeling is more flexible and free of issues than NURBS / solids modeling. If you’d like to know the details why I think so, I’ve written down the advantages of SDF modeling here.