The type of photography that you are planning to use is an important decision factor. Here we have evaluated the Sigma 18-250mm F3.5-6.3 DC OS HSM and the Canon 18-135mm f3.5-5.6 IS for their suitability to various photography scenarios:
Size and Weight Comparison
The weight of a lens is as important as its external dimensions, especially if you are planning to hand hold your camera and lens combination for longer periods. Canon 18-135mm f3.5-5.6 IS weighs 455g, 27% (173g) lighter than the Sigma 18-250mm F3.5-6.3 DC OS HSM's weight of 628g.
Lens Mounts
The Sigma 18-250mm F3.5-6.3 DC OS HSM has the Sony Alpha lens mount whereas the Canon 18-135mm f3.5-5.6 IS has the Canon EF-S lens mount. Some of the latest released cameras that are compatible with these mounts are Sony A68, Sony A77 II and Sony A58 for the Sony Alpha Mount and Canon T8i, Canon 90D and Canon SL3 for the Canon EF-S Mount.
Sigma 18-250mm F3.5-6.3 DC OS HSM is also available in Canon EF , Nikon F (DX), Pentax KAF3, Sony Alpha, Sigma SA, Canon EF, Nikon F (DX), Pentax KAF3 and Sigma SA mounts.
Canon 18-135mm f3.5-5.6 IS is also available in Canon EF-S mount.
Focal Range
Sigma 18-250mm F3.5-6.3 DC OS HSM has a focal range of 18-250mm and 13.9X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 27-375mm when used on a APS-C / DX format camera. Canon 18-135mm f3.5-5.6 IS has a focal range of 18-135mm and 7.5X zoom ratio which has an effective (full-frame 35mm equivalent) focal range of 28.8-216mm when used on a APS-C / DX format camera.