sigma lens or canon lens

[mazin]

Hi every body ...... I wana ask a question for professionals
If I get canon 50d or nikon D90 ,what are the best lens with hi zoom APS-C/DX lens :
Sigma 18-250mm F3_5-6_3 DC OS HSM;Nikon 18-200mm f-3_5-5_6G IF-ED AF-S VR DX;Sigma 18-200mm f-3_5-6_3 DC OS HSM ;Canon EF-S 18-200 mm F3_5-5_6 IS
I wana the lens of hi quality image
thankssssssssssssssssss

 

[Big Mike]

There is usually an inverse relationship between zoom range and quality. On >10X zoom lenses like these, the quality isn't going to be high.
I've heard that the Nikon 18-200mm is OK, but it will still be pretty poor compared to something like a 70-200mm.