Non-stabilised superzoom lenses have really had their day. The combination of long telephoto focal lengths and fairly small maximum apertures of around f/6.3 runs the ever-present risk of camera-shake.
So while the ageing non-IS version of the Sigma 18-200mm and Tamron 18-200mm are quite cheap, they're not the best value.
On a tight budget, we'd forego extra telephoto reach and buy the Sigma 18-125mm DC OS HSM, which offers impressive optical quality and comes with 4-stop stabilisation and Hypersonic autofocus.
Both Canon lenses are good rather than great, with basic feature sets, considering their fairly high prices. Image quality is only average, but at least you get the advantage of in-camera corrections for peripheral illumination on most of Canon's latest cameras, plus the option of lens tuning in Digital Photo Professional.
The biggest hitters in the group are the Sigma 18-250mm DC OS HSM and the new, radically redesigned, Tamron 18-270mm Di II VC PZD.
For compactness, light weight, impressive image quality and class-leading 15x zoom range, the Tamron wins out.