i had the same dicision to make, and couldent really find a differance, the tanaka was good stock, but the kjw was cheaper and only needed a few easy upgrades like hop up and barrel, and would still be cheaper than the tanaka, but im sure you have heard of you get what you pay for, and that was my basis on buying a tanaka, im not saying the kjw's are terrible, but im sure little things on the tanaka are built a bit better or have better tolerances. if you have the money to get a tanaka, and still have enough for a scope bipod, etc, plus upgrades, sure. but if your on a budget i'd say get a kjw by the time you upgrade it it'll probably be on par with the tanaka but still be a bit cheaper, unless the tanaka is a pre-ban, then spend the extra for a pre-ban. i know you can mod the kjw to fire up to 600 fps or something, but thats pretty dangerious, and if you go to differant fields, it's nice to have the ability to lower your velocity or increase it to play on that patricular field....
summary...tanaka vs . kjw tanaka if u have money for everything...on a budget? get the kjw and upgrade it, still cheaper than a tanaka and will be on par
pre-ban vs. kjw? get the pre-ban forsure, its just a hastle to mod the cylinder on the kjw to shoot at field limits, and if you get lazy and i know i probably would have if i got the kjw, you'll probably just shrug it off and not lower the fps for field limit, and possibly get someone hurt.