(This is really the only part of your question that isn't off topic as a shopping question or primarily opinion based.)
Are there any advantages of the IS and better focus motor in photography besides the more stable handheld photography?
Yes, there are benefits to using IS when taking still images with a handheld camera. It allows one to use slower shutter speeds than would otherwise be the case before blur due to camera motion becomes noticeable in the resulting image.
Regardless of how good your handheld technique is, you can stretch that good technique even further using a lens with good IS if your subject is stationary.
On the other hand, if you are using your camera secured to a tripod for very long exposures or are using it to take photos of moving subjects in low light, IS doesn't do anything at all for you. IS has no effect whatsoever on subject motion.
For more about IS and its uses for still photography, please see:
Is image stabilization a necessary feature for wide angle lenses?
What is more important, f-stop or IS
What reduces blur from camera movement more: large aperture or image stabilisation?
What's the difference between using an Image Stabilization and not using it?
The same is true of STM: It is an advantage if you need faster, smoother, and quieter autofocus. It's not an advantage at all if you shoot astrophotography or landscapes (or anything else) and manually focus the lens.
STM vs Non STM lenses for still photography?
What does STM mean on a Canon lens?
Why is the battery required to manually focus through the viewfinder?
There are other differences between the two lenses as well. How applicable they are to a user depends on what the user desires to do with the lens.
For example, the non-rotating front element on the STM version is useful if you plan to use polarizing or graduated filters on the front of the lens. It's a total non-factor if you don't. And typically, those who use such filters are more advanced photographers that have long since moved on to nicer lenses than an 18-55mm kit lens by the time they start using such filters.
For these reasons, we can't tell you if the extra cost is worth it for you. Only you can answer that.
For further reading, please see:
What advantages does the Canon EF-S 18-55mm III have over the IS II version?
Is there an IQ gain in going from Canon 18-135 IS to 18-135 STM lens?
