Originally Posted by Cactus
Some silliness in some posts above.
Threadless headsets allow cost reductions in bicycle manufacturing. One fork (cut to length by the LBS) works for all frame sizes, and never needs to go through a threading operation.
Quill stems are easier to set up and adjust to the rider. They also allow the handlebar to be higher (yes there are cludges to make threadless stems higher).
Headset/stem weight is a red herring. You really have to consider how much difference it will make in your ride. Generally, the honest answer is none.
As to twisting a handlebar so that either end shifts 2", well that's just not goinjg to happen unless something is broken. And, that might be the rider. There is no way, standing in front of a bike, that anyone can stablize the steerer tube enough to tell of motion is caused by the stem bending, or the more likely culprits of the whole bike moving, or worst case, the front wheel being soft and bending.
There is so much silliness of stiffness that is just so much marketing brainwashing.
So, either headset type and stem works just fine.
The stiffness issue isn't just about marketing. Look at the physics and materials. A quill stem is a narrow tube that has to fit inside the steer tube of the fork. Being of a narrower diameter, it is made stiffer by using thicker walls but it still can't match the stiffness of the steer itself. A threadless stem is mounted outside the fork steer and is composed of tubing itself so it now has about the same wall thickness as the quill but it has a much larger diameter. That in itself makes the stem stiffer than any quill can be.
Because most threadless stems also use the same diameter tubing to connect the bars to the steer tube, they are stiffer in terms of twisting force also.
Threadless headsets may be cheaper to use during manufacturing but that doesn't mean that they are not a superior product. I have much experience with both and I can tell you that to make adjustments in the field is trivial with threadless. Making adjustments and installation at home is also trivial. Installation from bare fork to test ride typically take less than 30 minutes including cutting the steer tube. Installation from bare fork to test ride using threaded headsets typically take 2 to 3 times as long.