Both the KS2 Spring and KS3 Spring are great cobra bag springs which I designed to be used in various ways. They are both DIY compatible as well as being the key component of the Kewlus Modular Cobra Bags. One way to sum up the difference is to say that the KS2 Spring is an entry level spring for casual training, while the KS3 Spring is a top level cobra bag spring designed to withstand daily punishment. However there's a bit more to it...
If you want a cobra bag that will be used by a number of people of different heights, including younger fighters, then I would recommend the KS3 Spring. Not only will it give you better overall performance, but it is also longer, giving you extra maximum height. It can also plug directly into the Multi Base for a short version of the cobra bag that can be used by younger fighters, or set up as a horizontal or upside-down reflex bag. The KS3 is also made from a more resilient steel, designed to last much longer.
The KS2 Spring on the other hand is ideal for someone who is new to cobra bags and would like to get started on a smaller budget with a view to upgrading down the track.
Here's a video where I demonstrate some of the key differences: