Taking this site as an example, “home working” is a pretty important keyword. I’d love it if I could be at the top of Google for that phrase. However, despite pouring heart, soul and a great many man-hours into creating a useful resource for home workers, I’m not yet anywhere near the first page of Google’s search results. (I’m actually languishing depressingly on the 11th page of results at the time of writing!)
So which tool should you use? The simple answer is ALL of them!! If the data from several tools suggests that a keyword may be a good keyword to target, than you should feel fairly confident that it is. If your results appear to be contradicting one another for a particular keyword, then you may want to be hesitant in including that keyword in your selection. SEO professionals should never rely on one resource for research. No one tool is going to be 100% accurate and you stand the best chance at making the correct strategic decisions by using a variety of sources.
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.