Never tried the KJW M9, but I looked at reviews for both of them (having to have read at least 10 each, and looking around for any technical errors that the guns may have had).
I have to admit, I stuck with the HFC M190. More replacement parts, high velocity, works like a charm with Marui M9 mags, etc. Only issues are no decocker in the safety, which is an unrealistic yet good point for me, and the trademarks (But it's for shooting, not for showing off).
Either way, other comparisons showed them on par, and 2 reviews done by the same guy on each gun gave them the same rating in each category.
However, another review on Arnies had the KJW M9 having rounds rolling out the barrel every once in a while, and a BB ejecting like a casing once (Yet, as AvTomAt kindly pointed out, it may be a lemon).
Either way, I can't tell you really which is better. They perform the same, but each has their high and low points. Your choice, review on both can be found at
www.arniesairsoft.co.uk .
Cheers, and good luck.
Nate