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.
AVCHD appears to just be h.264 with some constraints. Mainly on framerate and resolution. It looks from the wikipedia page that early versions mostly favoured interlaced encoding, unless you drop down to 720p.
The wiki page doesn't say, but I assume it's H.264 4:2:0 8-bit, not Hi10 profile or something. I also have no idea what quality the hardware encoders in camcorders usually manage. If you're generating AVCHD video on a computer, you could do it with x264 and get better quality per bitrate than ProRes. (if 4:2:0 8bit is sufficient).
This might be a poor choice if you need fast random access to non-keyframes, since a lot of the compression advantage of h.264 is from inter-frame compression. If you're just going to decode it in-order, then it's not an issue.
Note: AVC is h.264. (just like HEVC is another name for h.265). As always, a bad h.264 encoder can be standards-compliant but make your video look like crap, which is why I always make a point of recommending x264 for encoding.