i had the opportunity to use both. sigma 18-200 os hsm was my first upgrade lens.

im presently using nikon 18-200 vr.

my comment. i shouldnt have sold the sigma lens for the nikon hahaha.

sigma is sharper, in my opinion, than the nikon specially on the 200 end.

and has the macro option though at 1:2 or 1:3, still theres the option.

and the colors are better.

but the nikon has a big edge on fit and finish.

but i would still go for the sigma because of the image clarity and for the price a bang for the buck.