I've used both and like both. Like all keyword r taiwan email list esearch tools, they have their pros, cons and quirks, as does Google's. The most important thing we advise clients about when using any of these tools is to look at the demand relation between terms, rather than the actual numbers. It's less about what X and Y is, and more about X being 3 times Y. While you can try to use these numbers to approximate actual traffic potential, that's much more advanced and beyond this post.
For this test, I stuck just to Keyword Discovery and Wordtracker's publicly accessible free versions. In some cases, that required multiple searches (plural versus singular, possessives with and without an apostrophe) to match up against the AdWords export which was made from just two searches; "womens shoes" and "womens boots." Their free tools pull from their regular databases, but only return 100 results. So to be fair to them, some of these challenges and the number of results matching up to the Google results would probably be different via the full versions.