I've tried both and like Cornerstone better, mainly because it has a far superior file-diff interface.
With Cornerstone, you can double-click on any file in your commit-list and it instantly brings up a diff of your version vs the repository version. This makes it very easy to quickly code-review your changes before checking in.
With Versions, as far as I can tell, you have to hit Ctrl-D, then hit Compare, and then wait a good few seconds before it loads up the diff tool.