I have two Kayfun 3.1 clones, the HCigar and one that appears to be Tobeco. Both are fine and vape great. The things I don't like about the 3.1 model is the size and weight (they are larger and heavier than the Kayfun Lite). They also have the fill holes on the side, which is OK but I actually find the Kayfun Lite and especially Lite Plus much easier to fill.
On my 3.1s, one has the "one way valve" hole (you push a needle point bottle in there to release a spring/ball bearing system which is supposed to seal after you take out the needle), and the other has a removable screw with a little hole where you fill with a needle bottle. They both work, although the "one way valve" doesn't always work properly.
The new Kayfun Lite Plus has an enlarged hole in the bottom for filling which is "funnel" shaped, and a screw with an O ring to seal it. I love this since I can fill it with any bottle, the little "funnel" guides the juice into the tank and the screw seals it well so it doesn't leak.
Oh, one more thing about the 3.1s, they have a drip tip that screws into the tank instead of a 510 type drip tip. One of mine has a screw in adapter so you can use a regular drip tip, the other didn't come with an adapter so I'm stuck with the metal drip tip that it came with. I really don't like metal drip tips.
The build decks on all of mine are very similar, if not identical, and all are easy to build on.
I have the EHpro KFL plus and love it, and the "Black Edition" looks like an excellent product also.