Hi Sing Liang!
The Lee big Stopper is a good investment for real long exposures. The NDX 400 IS good too. But it doesn't feel like a 9 stop filter. It was almost like a 7 stop filter. What seems to surprise me was, the Lee seems to exhibit a bluish colour cast tint on my images. But it can be solved by pushing the colour temperature to 7000K and above to achieve the exact colour as on scene. The lee is all about convenience and time. You don't have to remove the ring adapter and holder just to screw on the 77mm Hoya then put it back altogether again...
In short, I'd say BOTH are good for image quality, and more or less give the exact colour on scene with little colour casting. But for colour rendition, I'd go with the Hoya. For extremely long exposures, B&W photography and convenience, I'd go with the Lee. Price wise, Hoya is $120 from tkfoto. The Big Stopper is a wooping $200.
Ultimately, just make sure you're shooting in RAW, so that'll be easier to fix minor colour casting and its much easier converting it to B&W.