Good question, and I'm asking it myself. The proposition of RSTuner Kit and the like as I see it is that while it is more expensive,
1. You get to change maps as often as you like and you can return to the stock one if there are any issues with the tuned map
2. If there's an improvement to the map, they will send you the updated version and you can upgrade free of charge
3. I often do not trust the small local shops. They will not tell you how they "developed" the map and I am often suspicious whether it has been tested on the specific model at all.
4. you also get a complete OBD scanner and data recording solution that works very well.