From Google Desktop features page:
Google Desktop is a desktop search application that gives you easy access to information on your computer and from the web. Desktop makes searching your own email, files, music, photos, and more as easy as searching the web with Google.
Lenses are elements of the Unity Dash that provide a UI to search both the web and application data.
In particular, how are the privacy policies and exposure of any personal data to either Canonical or Google different? I have not used either one of these and my first step would be to find out more about what personal data is exposed.