Golden Arcade at Sham Shui Po may have the best prices as is traditional. They will often sell you copied games but you could download them yourself I am told.
Apparently the US version is better because it can handle Japanese games whereas the Japanese version has trouble playing some non-Japanese games. That is the main reason why the US version is often more expensive.
though apparently the Japanese version outsells the US version by a large margain. The price difference is almost 1K.
BTW, the US versions was available for abt 100-150 cheaper at other shops in Golden Arcade - but the total cost of modded machine+additional set of contollers+adapter worked the cheapest at the place I bought at.
So ask around because I got quoted a wide range for each: the machine, controller+nanchuk and modding.