Hi Steve, both applications have their pros and cons.. I tend to use iWork if the document is going to be owned by me completely, and when office collaborating with others from office, I tend to use MsOffice.. I prefer iWork since the output has a significant edge in quality.. ..features-wise, I'd probably say that MsOffice has had much more time to mature (though I dont like the font-rendering by MsOffice on a Mac)..
Re. Mountain Lion, I dont think I have an option to do a clean install - I wish I could, but till Apple releases a Mountain Lion USB bootable, I dont think I have an option..
The CPU usage issue is mainly with Numbers only.. Other apps seem to be fine.. (I tend to use Firefox more anyway, so dont know if the issue with Safari exists or not)
Hope that helps,