I recommend getting a genuine KLF+. Intaste, an excellent vendor in Germany, sells it for about $85, and the US distributer, Vaperev, sells it for about $100, see links below.
The Kayfun Lite Plus v2 is a classic RTA and will probably always be the standard by which others are judged. It is effective, beautiful, and
simple. I disassembled and cleaned my K4 yesterday, and it was a chore. The KFL+ can be fully disassembled by experienced hands in about 2 minutes.
I have a Russian 91 and don't really care for it. The finish is (in my opinion) ugly on anything but a black mod. Mine tends to leak lightly from the fill screw, and I strongly dislike the 510/center post design. It was also quite dirty out of the box and came with a needle bottle that also leaked whenever I shook up my juice in it. Maybe I just got a lemon, but since they both cost about the same, and since you already own an R91, why not get a KFL+?
Kayfun Lite plus - InTaste
Vaperev Shop - Kayfun Lite
(BTW I don't mean to offend with my comments against the R91. If it works for you and you like it, great!)