Chrispoole,
are you planning on texturing as well with your models? If so I would suggest both but also buying a rendering software or I think a better way would be Moi and Modo. If minor animation by moving things around then Modo will work for that as well. If your thinking about doing major animation then maybe Lightwave, Softimage, Max & Maya of course depending on budget what your expecting to do.
When modeling I think of Moi as my mechanical building software and Modo, Lightwave, Silo, Hexagon and so on as more of an organic modeling tools depends what your going to spend your time doing. The other programs do a good job in mechanical as well but I feel they have limitations in some regards for mechanical and take a lot of time when doing mechanical. Plus Moi has a feel to it that doesn't match any of the other modeling programs with the use of ease. That is my 2 cents on this subject good luck in however you decide.