Use the Google Adwords Keyword Planner to find new and related keywords, but ignore the search volume data! The search volume data in the planner is really only useful for keywords that you’re actually spending money to advertise on. Otherwise, these volumes are not reliable. While not really helpful to decide which keyword is most used by your potential audience, Google Adwords Keyword Planner makes a useful tool in coming up with ideas for potential keywords!
The WordStream suite of Keyword Tools is an integrated and actionable set of keyword tools designed to help search marketers with keyword search, keyword suggestion, keyword grouping, PPC keyword research, keyword analysis, long-tail keyword research, and negative keyword discovery, going beyond the capabilities that a typical free keyword tool can offer.
It would be great if you can provide comparative analysis of this tool with Google Keyword Planner. Since you’re using this tool so you can provide best comparison. One differentiation which I understood is that this tool works for yahoo and bing as well whereas Google keyword planner works only for Google but Google being a dominating search engine, gives the highest returns, so only this reason may not be enough to buy it.