One good reason to look at a system that uses NURBS data, is it will be analyzing a true NURBS surface and generating off of that.. If your package requires STL data, then everything will be linear cuts.. Although with MoI, you can dice up your model well and simulate curvature well enough that you dont need to run out and buy something different.. You would look for something different if the "toolpath strategy" in the package was somethinhg you needed.. Like an equaldistance offset or a Z level finish, or a 4th axis or something..

Then as you progress into cnc, your needs will change, like the abilities to control the tools entry and exit from the material more...Or to control different types of machines and various other aspects.. This is where you start to move up in the price range. You dont want to run your $400,000.00 Haas with a $200.00 software :o