I have a Nikon D-80 and I wish to buy a telephoto lens

Hi,
Sigma does make plenty lens option in both ranges you are looking at, you may want to consider a zoom lens that will work for you. What type of photography do you do? That will help determine your lens selection choices. keep in mind that it is more important to purchase the highest quality lens you can afford rather than several lower quality versions. If you are looking at a 300 mm lens to capture sporting events or nature photography you will have determine is it going to be daylight or low light because a lens that works in lower light situation will cost many times more than one with less capability. If you like the wide angle to take photos of interiors the same goes as far a price. You may also want to consider your camera's brand of lens, the Nikkor lens and even look a the VR lenes (vibration Reduction). There is an 18-200mm1:3.5-5.6 that is pretty versitile and will work in a lot of situations from portraits to full standing poses and take great interior shots. The VR lenses are a little more pricey but allow you to hand hold the camera for pretty slow exposures. I know that is a lot to think about but it's important not to waste you hard earned money on lens combinations that won't be useful for what you want to shoot.