A:Answer Besides the appx $200 price difference, the 55-300 lens is strictly for use with digital cameras with DX sensors only. DX cameras have a smaller size digital sensor than an FX that uses a standard 35 mm frame (24 x 36 mm). Because the lens physically cannot 'fill up' the FX frame it is listed only as a DX (and thus there is no 'equivalent' for FX/35mm).
The 70-300 (that I own) is physically a larger lens than the 55-300 and is designed to work either with a DX, FX, or 35mm film format. Thus, while it is listed as a 70-300 for the FX/35mm format, it gives the equivalent performance of a 105-450mm for the DX frame if you're familiar to what things look like through a 35mm/FX camera. The Nikon website comparsion between the lenses will show you the range in minimum/maximum angle with each lens, noting that both the DX and FX/35mm values are given for the 70-300.
