The advantages that I have seen include first and foremost the access to far more RAM within the 64-bit working environment. This allows SPM more workspace for the GUI and will make the operation of even older 32-bit SPMs smoother. If you need to work with data files larger than can comfortably fit within the older 1 GB limits (due to Microsoft limitations) you will have far more flexibility under Windows 7. Further, you can now upgrade your license to push to the limits supported by your hardware. At Salford Systems our best in-house server is equipped with 256GB RAM and we regularly leverage all of it when working with larger data files. My MacBook now has 16 GB RAM of which I allocate 8 GB to Windows 7 and the ability to work with substantial data sets has been a joy.
Windows 7 is also a fair bit more responsive as a working environment for anyone with a newer multi-core machine. The higher versions of SPM 7.0 can also leverage multiple cores for faster run times but you will need Windows 7 to take advantage of this. Good luck with your upgrade!
If you've also compared performance across different operating systems, let us know your experiences and takeaways in the comments!