Update: I’ve now been using Long Tail Pro for 5+ years. There are now many different keyword research tools and lots of people prefer some of the newer tools. Admittedly – Long Tail Pro had some hiccups and went through some rough times as they transitioned ownership and introduced the newer cloud version. But – all of the bugs and kinks have been worked out and Long Tail Pro is now better than ever! I’m happy and proud to be able to continue to use and promote Long Tail Pro as my keyword research tool of choice.
SEMrush is a very useful tool for both researching competitors when starting a site or for growing an established site. I really like to find weaker niche sites that still seem to be ranking for lots of keywords; SEMrush helps me see what they are ranking for and what I can potentially target. You can also see what keywords you’re on the cusp of ranking for with your established site - another very useful feature.
Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website. In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC). I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.