Notes on "star trackers":
When using the Pentax astrotracer functions, there are some limitations. Wide angle lenses will show some blurring of star images towards the outer field because the motion correction is being done on a flat plane in the camera by moving the sensor. This motion isn't correct for all of the stars in the field so the further the stars are from center field, the worse tracking will be. There are limitations on the time tracking can be done this way because the sensor can only move so much.
Using a tracking
mount overcomes all the aforementioned issues because the entire camera is being moved in a circular fashion to track stars and sky objects. Widefield lenses can be used and will be sharp to the edges (within the lens IQ limits). Exposures can be much longer and are only limited by camera, batteries, mount calibration, weather, and how long it's dark. The downside of a tracking mount is its cost and the calibration it needs when setting it up. Of course, it also takes up additional space beyond that of just a camera and regular tripod. It also needs a good tripod so it can be extra vibration free and steady with the load. Tracking mounts are limited in what they can accommodate load-wise, but more $$ can get mounts with greater load capacity, right on up to world class telescope size
The choice depends a lot on what kind of sky shooting you want to do. After all, you can get some pretty neat looking pictures with no guiding - just long exposures and the right setting (star trails).