Sigma 18-200mm vs Tamron 18-200mm lenses for Nikon D40x?

Are these good quality lenses? I just got the Nikon AF-S 55-200mm VR lenses,

http://www.bhphotovideo.com/...ikkor.html

and I'm thinking about returning it to get either one of these…

http://www.bhphotovideo.com/...II_DC.html

http://www.bhphotovideo.com/...Di_II.html

Are these lenses good? Are they better than the Nikon Lenses? And if yes, which one is better, Sigma or Tamron?

Added (1). And I was looking at this too,

http://www.bhphotovideo.com/...ED_VR.html

Yes, they are good lenses. Trading it in for the wider range, 18-200mm vs 55-200mm, is a good idea. They are not better than the 18-200mm Nikon lens, the Nikon is better. I own and really like the Nikon 18-105mm lens, but have recently replaced it with the Tamron 18-270mm PZD lens. I really like the extra reach and have found the quality of pix meets and exceeds my expectations. If you are budget driven, the Sigma 18-200mm is an excellent buy. If you want a really wide ranging single lens, then I would recommend the Tamron 18-270mm PZD lens.
http://www.bhphotovideo.com/...Di_II.html

I had the Tamron 18-200 a little while ago and was pretty happy with the results. As long as you understand that you are paying for the convenience of being able to use 18mm to 200mm without having to change lenses because the IQ is decent but not great. I used it primarily when changing lenses multiple times was not viable.