Do I need HalfTime if I already own TimeShaper?
HalfTime is entirely dedicated to half-speed effects. TimeShaper can do the half-time/half-speed effect and much more, such as scratching, glitching, stuttering, repeating, speeding-up, reversing, etc.
As far as the differences between them when it comes to half-speed effects, HalfTime has two fully adjustable Smooth modes, Percussive and Sustain, which you can also set to long values for creative FX (FX and Swell modes). TimeShaper only has the Percussive mode, with a short, fixed smoothing time.
The biggest difference, though, is convenience. HalfTime's interface makes it straightforward to create a slow-down effect (no setup – just add the plugin). In contrast, TimeShaper has an endless range of applications and is thus much deeper and more involved.