I use both Google Drive and Dropbox. I do that to separate my personal files (in Dropbox) and work files (in Google Drive), so there is not mixing up the two. One reason I use Google Drive for work purposes is its easy of sharing files. It just go easier to share files in Google Drive.
Google Drive added the feature to show profile pictures of the collaborators. This helps to ensure the right person sees the files. If you share with more than two people, GDrive now suggests related people with whom you frequently share the files. Neat!
Dropbox moved up to version 3. Version 3 includes the rewrite of Windows and Linux user interface, file identifiers, and better support for Mac menu bar icons. The file identifiers are an interesting addition. They enable Dropbox to detect when the files have been moved or removed. The blog states that this feature will be developed in the future releases.
I am happy to see continuous improvements with both services!