Both the Canon EF-S 17-55/2.8 IS USM and Sigma 17-50/2.8 EX DC OS HSM feature optical stabilization via a movable internal element group. In operation sensors in the lens detect movement (angular rotation about the horizontal and vertical axis) and move the stabilization optical group in such a way as to keep the image stable on the sensor.
This movable optical group is somewhat "loose" when power isn't applied to the lens, so if the lens is shaken you will hear a rattle. This is normal and not a sign that something is wrong with the lens. I found that the rattle on the Sigma 17-50/2.8 EX DC OS HSM was a little more pronounced than the rattle on the Canon EF-S 17-55/2.8 IS USM.
I did notice that the Sigma 17-50/2.8 EX DC OS HSM appears to make a very slight "hum" when attached to the camera, as though there's something always spinning inside (a gyro sensor maybe?), while the Canon EF-S 17-55/2.8 IS USM was silent. When the shutter is 1/2 pressed the Sigma noise changes slightly and the Canon lens also starts to make noise as it's sensors activate. During operation (shutter 1/2 pressed) the Canon lens makes a slightly louder sound than the Sigma, though both are pretty quiet.
Both lenses have a rubberized zoom ring and both turn in the same direction, with 17mm on the left and 50/55mm on the right.
The Canon EF-S 17-55/2.8 has focal lengths marked at 17, 20, 28, 35 and 55mm, while the Sigma 17-50/2.8 is marked at 17, 21, 28, 35 and 50mm. Both lenses extend in length when zoomed out from 17mm (i.e. they are not internal zoom lenses).
The image below shows each lens zoomed out to its maximum focal length. The Sigma is on the left, the canon on the right.
The Canon EF-S 17-55/2.8 IS USM I had for review was not a new lens and was on loan from Canon. There was noticeable zoom creep, i.e. when the lens was held vertically the zoom setting would slowly change. Talking with other owners of this lens it appears that this is not a normal situation, so I'm assuming that this (used) lens was in need of servicing. Zoom creep was reported by a couple of users, but most did not see this problem.
The Sigma 17-50/2.8 EX DC OS HSM lens was brand new, it exhibited no zoom creep and the zoooming action was stiffer than that of the (used) Canon lens.
The manual focus ring of the Canon EF-S 17-55/2.8 rotates by about 90 degrees going from infinity to the closest focus distance (35cm). The Sigma 17-55/2.8 focus ring only rotates though about 40 degrees going from infinity to close focus (28cm). So it's possible to manually change focus distance on the Sigma lens slightly faster than on the Canon, but it's easier to get precise focus with the Canon since it's easier to make small changes in focus distance.
The Canon 17-55/2.8 has FTM (full time manual focus) which means that focus can be manually adjusted at any time without having to physically switch the lens from AF to MF. Though the manual focusing ring is active at all times, it does not spin during autofocusing. By way of contrast, the manual focusing ring of the Sigma lens does spin in AF mode and in order to manually adjust focus the AF/MF switch must be set to MF. Though you can turn the focus ring in AF mode, it's very stiff and obviously not meant to be used that way.