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.
From What exactly is a "lens"?
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.
Regarding the privacy policy, Lenses in Unity do not send any personal information to Canonical. The dash displays your recently used files, folders and applications based on Zeitgeist which creates a database in your own home folder. In this way you have complete control over your personal files, folders and application usage.
Some Lenses like the AskUbuntu, youtube (still in development) and Reddit lens performs a search query on this website. So it doesn't in anyway infringe security breach or any privacy issues.
This is the same case for Google Desktop. You can read their privacy policy here. They do not store the indexed data in their servers without your permission.
Hope this answers your question.