The problem with KW Difficulty is that they should take others metrics, specially the one that relates with links. Most people believe that the best link database is Ahrefs, therefore something that analyze the SERPS using the Ahrefs estimation system shall be the best. Which is so? Ahrefs itself has this tool, therefore we must believe that Ahrefs keyword difficulty index is the best.
Don't trust those metrics, I like KW Finder because the UI makes it simpler to see each KW and inmediately after, the Serps for that KW.
I recently bought Keyword Revealer for the price to see if this could satisfy and replace KWFinder but it isn't as polished. It takes a week to get the report for each KW. So KW Finder is the killer in terms of usability.