What is the difference between AVCHD and ProRes in terms of quality? Depending on the project and the camera of course I often have to work with ProRes or AVCHD codecs without any thoughts. I assume that the code and the compression algorithm certainly is very different. Therefore I would like to know what is the difference between AVCHD and ProRes in terms of quality for a solid workflow?
You focus on quality in your question, so the answer is clear: ProRes (in various flavors) offers superior quality. There are other reasons you might choose AVCHD, but they don't relate to quality.

AVCHD is generally speaking a distribution format, while ProRes is more an acquisition and intermediate format (it uses all I-frames). ProRes also comes in a 4:2:2 /10 bit flavor, where AVCHD is 4:2:0 / 8 bit.