In reply to spidermonkey09:
If you climb in places where the small cracks are featured and/or irregular, or if you have to deal with pin scars, then small head width matters a lot (a cam that you can't get in isn't worth much), and C3's are way narrower than Zeros.
Flexibility may not be an advantage either. Here is a comment from BD on the subject (but note that the references to loads relative to planes of movement seems to have confused parallel and perpendicular):
"Not only do we have tests to show that it (the ‘stiffness’ of the stem) is not a problem, but we have tests that show too much flexibility is a problem in certain placements. Think about how cams work in a perfect placement: you pull straight down on the cams, that force is converted to a rotational force on the cams, and the surface of the cams push against the rock surface. Now place a highly flexible cabled cam in a vertical bottoming crack (the stem is sticking out horizontally). Should be good right? Think again about how the cams need to work. Pull straight down on this placement and you’ll notice a large component of the downward force is acting parallel to the cam surfaces. This is not how the cam is designed to work. The cams will not push against the rock surface without a force perpendicular to the cams plane of movement. In short, it stops camming and acts like a nut placement. You need some torque on the placement to convert the parallel load to more of an outward one that can act on the cams. This torque is provided by the stiffness of the cable, or the length of the shank of head terminal, or some combination of both. In testing, the high flexible cables did not generate the torque necessary to hold the falls and the units slid sideways out of the placement at very low loads. We even made C3s proto types out of softer cable, only to watch them fail our bottoming crack test because of this issue."
This quote extracted from an extensive review by John Griffith at
http://www.alpineexposures.com/pages/black-diamond-c3-gear-review .
Personally, I've used both, and once I had C3's, the Zero's disappeared from my rack. The C3's fit where nothing else, including Aliens, will go, but Zeros in particular are way out of the running with a much bigger head width.