Going back on topic again. (had to quit flirting first)
I don't know the differences in the various kayfun models yet and not sure how to compare that to a Sophia.
The regular kayfun is larger, has side fill port and adjustable air control. The KFL has a bottom fill port and no adjustable airflow control, the KFL+ has the bottom fill AND airflow control. The Lites are both smaller than the original kayfun (hence "Lite"). The Russian and Russian 91%...well, technically clones, but of equal (some say better) build quality. From what I understand, the Russian folks that originally designed the Kayfun had a falling out. The guy that owned the rights went to Germany and started building Kayfuns, his disgruntled partner took the design to Kebo and they put out the Russian. Here's a link to a pbusardo video where he reviews the KFL and the Russian 91% side by side. The KFL+ came out after that, I think.