Hello..
I have Nikon D5100, I currently have 50mm lens & 18-55.
I want to get a new zoom lens, I am hesitated between the Nikon 55-300 VR and the 70-300 VR.
The 55-300vr is about 1/3rd smaller and lighter with metal mount. But optically considered neck to neck with the 70-300vr. Some say the 55-300vr is slightly slower to AF or lock but negligible. Also to consider is the Tamron 70-300 USD VC for about the same price.
I wouldn't have too many reservations using it and seeing performance on par with the 70-300vr, There may be a couple of situations where the 70-300vr would edge over but not many. As the 70-300vr is better constructed throughout. Can be used on Dx and Fx cameras where the 55-300vr is a Dx lens. If doing a lot of zoom needs for sports,wildlife would opt for the 70-300vr. Also if ever down the road upgrading the zoom. Then there will be more buyers for it then for the 55-300vr due to it being for full frame and crop cameras.
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I have the 70-300 and it's great. As orb said the 70 is an FX lens which is a real big plus IMO. I chose the 70 over the 55 because according to Slrgear.com tests it was a bit sharper. I also looked at the tamron (which will auto focus on the d5100). It is about 100$ cheaper but I liked the performance of the Nikon lens better.