I love my K10 full-size. I've am planning to buy a 75% sized Keychron. My only complaint is that the Keychron K10 acts as an Apple keyboard under Linux and loses Function key abilities. An options can get keys F1-F12 working, however then the FN special key doesn't work. I wish it would just work like a regular Windows keyboard HID device. As for build quality, this is a great keyboard.
I wasn't entirely sure between the sets, there's not a lot of information as to why. The other sets never seem to be available so I was excited to see that these were available in Brown, once I got them I finally realized that these are for the K12 and K10 models and not the other models. I thought there was no difference either set is mechanical. But when I finally got it on my board I realized there is a size difference. Not entirely explained so I'm not very happy about that.