Welcome to the forum Ron.
The Nikon is slightly faster at the telephoto end at f/5.6 vs the f/6.3 of the Sigma so this means the Nikon would be slightly faster across most of the 18-200 range allowing you to get marginally faster shutter speeds.
The Nikon is better built, has a higher quality and slightly more effective stabilisation but costs quite a bit more so you will have weight out the advantages against the extra cost. Cost aside though, it is the better lens.
Like pqtips says (
) this is probably the wrong place for this question and you'd be better off posting in the Nikon or Sigma section.