I insisted on getting the lightweight Space Navigator. It is $100 and very basic with 2 buttons. I do use this a lot for orienting the model and it does save me time. I spend a lot of time doing this when evaluating designs so it was very easy to justify. I have used 3D mice for about 15 years.
What I don't like is that it is not a true 3D input device. Seriously, why is this not a real 3D device for gaming and other native 3D programs? 3D Connexion doesn't seem to be working with a unified architecture.
With the extensive use of the CTRL/ALT/SHIFT buttons in Creo, the left hand is becoming quite busy. I haven't tried but I would think that programming ALT and CTRL on the Space Nav would be helpful.
I can't justify all the buttons of the more expensive versions. I remember having to "develop" tablet layouts in a past life... and this had to be done for nearly every new version. Like mapkeys, it is a pain to loose all that effort.
Last, I contacted 3D-Connexion about their Space Nav and Space Nav for Notebooks. I bought the heavier Space Nav only because I had experience with it and the fact that the extra weight really is an advantage. I got the same sense from the rep I was corresponding with.
Hope that helps.