I have both of these softwares. They are both great tools to have. However, each of them will function differently. For content farret the content is limited to what is in the current database, although this is being added to over time.
With IAW it pulls the content from the web search engines, so this tool is great when you have to create content on your own topic, and not only use what is only available in a database like with content Farret.
Both tools have the function to help you spin the articles when completed. But, TBS is also another great tool for doing this as well.
So, based upon what type of content your needing to create should first be determined, and then this will better help you to determine which of these tools would actually do the better job for your own needs at the moment.
I invested in both, so that I could create content for article marketing purposes. But, having IAW helps me when I am needing content ideas for my own niche topics without being limited on the types of research it will bring back for me.
Hope my answer helps you out some with your decesion on what tool to select.