Kontakt is more of a sample workstation, while SampleTank is more of a sample player. There are more sample libraries available for Kontakt than SampleTank. In fact, SampleTank as very few 3rd party sound libraries besides its own developer (IK Multimedia) Sonik Reality.
Last I checked, Kontakt can import/convert a variety of formats, such as EXS24 and Akai, for example, while SampleTank does not. Someone might correct me on this.
If you like the sounds SampleTank offers, then there's nothing wrong with getting it. Just understand that you're really buying more or less a sample player for its own library, and not a "sampler."