HitTail uses the existing keywords people are using to find your site and automatically generates long tail suggestions for you each day or week (depending on your plan). It uses your proprietary keyword data to come up with more keywords your site is highly likely to rank for (low hanging fruit).

It appears that KWFinder asks you to enter a phrase and provides a list of long tail keywords. So it's more similar to LongTailPro or the Google Keyword Planner than HitTail - meaning it pulls from a single database of keywords they have compiled or otherwise have access to pull ideas from.