What lens should I use?

I was just looking for a Lens for my Nikon D3000, and I use it for spotting planes, I currently have 2 lenses an 18-55 and a 18-105 lens both from nikon but they don't have enough zoom. However my mom has a rebel xsi and has a sigma 18-200mm lens made by sigma. So I was wondering what a good lens is for spotting and should I get a Sigma?

You want at least a 300mm end on your zoom. If money is a major issue, look at the Nikon 55-300 VR. The image quality with the 70-300 VR will be noticeably better.
Here are several samples of 300mm images, some of which are airplanes, so you can see how much detail is possible.

For plane spotting you'll need a longer focal length than 200mm. Try 300mm. There are 55-300mm and 70-300mm lenses from various manufacturers for various prices.
Nikon, Sigma and Tamron all make lenses you could use. Go to your local camera store and ask them to show you the lenses they have in stock.
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