The new MK3 version wide-profile brakes are significantly more powerful than previous versions. This is due to their longer arms. Longer arms increase mechanical advantage. But just as important is their excellent adjustability. They now have spring tension adjustment for easier centering.
We've kept the traditional wide-profile design of the MK2. And they still have slots for up and down adjustment of the brake pad. This means they fit perfectly on more frames and forks; not all canti studs are in exactly the same place due to manufacturing tolerances or intended specification. There is also an adjusting mechanism at the end of the straddle wire for easier setup and for fine tuning pad clearance and wire angle. (You'll wonder why all canti brakes don't have these adjusters.) Finally, the brake pads are fully adjustable for toe-in and angle.
The graph above shows the mechanical advantage of the three models of the Grand Cru Cantilever brakes at various yoke heights. It's good for comparison, but keep in mind that mechanical advantage also varies with different brake stud location, rim width, distance from pad to brake, exact arm position, and brake lever type. Also, you can't reasonably get the yoke below 100mm, so ignore the far left section of the graph.
Should you order the MK2, MK3, or Zeste brakes? Compared to the Zeste, the MK3 and MK2 are easier to set up because they are not as sensitive to yoke height variations. They are also more traditional, and perhaps cooler, in appearance. But they have a wide profile that can snag on panniers or heels if your bike has short chainstays. The Zeste brakes have more power and that may be important if you ride a loaded touring bike in the mountains, but they cost more. They also have a low profile. The MK2 brakes are still very good and a super value now that they're on sale.