Been a long while since I rode a dual K brake setup but I used my GL setup yesterday . . .Both stop dramatically better than a stock single caliper /rotor. GL brakes will stop you better though. Better control and feel. The K setup has a good deal more "slop" built into the system with the brake arm pivot points, long lever arm and less robust caliper casting. Combine this slop x 2 and the feel and control suffer.
Other things to think about:
The GL setup self adjusts for pad wear.
They are both heavier than hell but I think think the GL stuff is a bit lighter (no pivot arm, slightly smaller diameter disc)
GL setup gives you larger diameter, stiffer (flex wise) forks. These forks are heavier too, no doubt
Probably less work and less money to install dual K discs
Either way its nice to use the GL's lighter weight alloy rim intsead of the stockers chrome steel one. This will help offset the weight gains some.
Neither of these setups perform like lightweight modern sportbike brakes.