Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Sigma 18-250mm F3.5-6.3 DC OS HSM is the longer of the two lenses at 89mm. The Tamron 18-270mm f3.5-6.3 II VC PZD with a length of 88mm is 1mm shorter. On the other hand, there is no difference between the max diameters of these lenses, both coming at 74mm.
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. Tamron 18-270mm f3.5-6.3 II VC PZD weighs 450g, which means it is 20g (4%) lighter than the Sigma 18-250mm F3.5-6.3 DC OS HSM which has a weight of 470g.
Below you can find the dimension and weight table of the 'SuperZoom Lenses in Canon EF Mount for size comparison purposes.
SuperZoom Lenses in Canon EF Mount for Size Comparison
Lens Mounts
Both Tamron 18-270mm f3.5-6.3 II VC PZD and Sigma 18-250mm F3.5-6.3 DC OS HSM has the same Canon EF lens mount. Some of the latest released cameras that are compatible with the Canon EF lenses are Canon T8i, Canon 90D and Canon SL3.
Tamron 18-270mm f3.5-6.3 II VC PZD is also available in Nikon F (DX) mount.
Sigma 18-250mm F3.5-6.3 DC OS HSM is also available in Nikon F (DX) , Pentax KAF3, Sony Alpha, Sigma SA, Canon EF, Nikon F (DX), Pentax KAF3, Sony Alpha and Sigma SA mounts.
Focal Range
Tamron 18-270mm f3.5-6.3 II VC PZD has a focal range of 18-270mm and 15.0X zoom ratio which has a 35mm equivalent focal range of 28.8-432mm when used on a APS-C / DX format camera. Sigma 18-250mm F3.5-6.3 DC OS HSM has a focal range of 18-250mm and 13.9X zoom ratio which has a 35mm equivalent focal range of 28.8-400mm when used on a APS-C / DX format camera.