Our design group just completed an evaluation of both, using weighted categories such as "Productivity", "Standard Features", "Ease of use/learning curve", "Step Compatibility", etc...
We reviewed SW and Inventor r5 (just released). The hands down winner was Inventor, although many categories were "ties". Some of the strengths of Inventor were:
-More intuitive user interface, with fewer dialog boxes and fewer mouse clicks. Gesture based sketching, glass box, "expert" and "learning" modes, more consistent interface with (far) less clutter.
-Better STEP file import: the colors and part names of the imported component were preserved, and the geometry was immediately useable. SW required the use of "FeatureWorks" to "repair" the same model.
-Adaptivity: the SW guys had a contrived example showing how they could do the same things that Inventor does: this is true only to a very limited extent, explore this carefully in your own eval...
Everyone should do their own evaluation, as each product has particular strengths. Don't let past experience with Mechanical Desktop cloud you judgement, Inventor is a completely different product, not a derivative.