KWA is KSCs export brand, since Japan doesn't follow "normal" trademark laws and therefore weapon trades are legal there but not technically "legal" elsewhere. Basically, KWAs are untraded KSCs, with a crappy metal slide to make up for it. KSCs also come available with metal, fully traded slides, much like my KSC USP.45 with metal barrel and slide with full beautiful trades. A lot of people selling "Shooters Design" slides are actually boffing you and selling you KSC traded slides for many guns, especially the USP.
Wei-Tech and HFC are the same company, but offer different models (both of similar quality though).
Then theres WA, the king of GBB, TM (you know that), the queen of GBBs, and then the rest, all the way down to KWC (not to be confused with KWA/KSC), which are teh suck but fun to get to shoot around.