Zombie: I have used keyword discovery (KD) and word tracker (WT) and there are advantages for both. I decided to go with word tracker because of the ability to setup projects and some of the other nice features that agencies can use. I also had some troubles with the reliability of KD which was not good for me or my clients and that was the deciding factor between the two.

When doing research also look at SeoDigger.com to get ideas of what the competition is doing.