Between the two you mentiioned, I would choose the Bowtech Black Knight 2. If you want maximum speed, then it is the bow that has it.

That said, none of those responding (including me) have any idea of your level of experience with a bow. The low brace speed bows are less forgiving of form breaks, or any "nervousness" in your shooting, yet with their aggressive force draw curves, no valley, and low brace, they tend to magnify anything less than perfect form. Can they be shot accurately? Absoutely, but there is also much more potential for disappointing days, frustration, and lack of confidence. If you are a rock solid shooter, then you don't have to worry about that.

All that said, one of the best shooters I've owned was a 2001 Bowtech Pro 38 Dual Cam. Fast? Yes, but very stable too with nearly 7" brace height (with the grip removed). I currently have a Patriot Dually that is also very fast, and has a 7 5/8 brace height. They also made the Pro 40 Dually, which was considered to be even more forgiving, and still quite fast. I've owned a couple of Mathews bows, and I'd have to say that I would much rather have an LX if I wanted a fast Mathews than the BM 2. The new Switchback looks like a much more shooter friendly bow than the Max also. JMHO, FWIW. The Darton Maverick Extreme and the Hoyt Supertec are two other nice speed bows.