1) SEMrush - I believe that among all the 3rd party software, SEMrush has the largest keyword database. Their search volume data is pretty accurate and aligns with the Google keyword planner. Also, based on the type of content that needs to be produced (i.e. informational, transactional, etc.), one can utilize different filtering options available in it.
In addition, you can dig into the paid side of search and find out what keywords your competitors are bidding on, and then leverage those keywords for your own organic benefit if you're not already doing so. Search Metrics does this as well, but I've found SEMRush to provide a greater range of keywords and they save more historical keyword data than Search Metrics.
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.