Lens 18-200mm

I have a Nikon D40 (I intend to change soon by a D90) and am researching a little on some lenses, I want to buy a 18-200mm and I got a doubt about the quality of other brands. The prices of Nikon lenses are much higher than the Sigma and Tamron. I am VERY far from being professional, I am indeed very amateur, but like a lot of shooting and I am with this doubt.

Would like your opinion about these three lenses:

Nikon Zoom Lens AF-S DX NIKKOR 18-200mm f/3.5-5.6G ED VR II
Sigma Lens 18-200mm f/3.5-6.3 DC OS HSM for Nikon Digital SLR
Macro Lens Tamron 18-200mm f/3.5-6.3 XR Di-II for Nikon Digital SLR

It's really worth paying almost double by Nikon, for someone who shoots as a hobby (much more enjoy shooting) and not as a profession?

I thank you for all feedback