Choose One, TAMRON or SIGMA?

I wanna asking to you guys who understand about DSLR lenses, i have Nikon D5100 and i decided to buy 18-200mm lenses but nikon brand is too expensive, so i need recommend to choose Tamron or Sigma and please describe about the specific differences of both, thanks and sorry for bad english

Neither. The Nikon 18-200mm is the only one that comes close to adequate quality, and when you upgrade your 16 MP body to one with 24 MP, even that may not look adequate.

If you own an 18-55mm, the next logical step would be a 55-200mm or thereabouts.

You'll also have to bear in mind that the lens you want may not autofocus with your camera, so do a little more research. An 18-200mm lens looks good on paper, but its weight and reduced performance may give you cause for regret.

If you already have an 18-55 lens. Do not buy an 18-200. It will be a waste of money. Instead, buy a 55-200 from Nikon. Very good value and the two lenses will cover the same zoom range with BETTER image quality and for a lower price. The Nikon 55-200 VR is a very good lens that can be found for under $150. Do not buy the 18-200 unless you are planning to sell your 18-55.

The order of preference when it comes to third-party is, Tokina, Sigma… Tamron.