Well, you gotta keep in mind that the 28-300mm may cover a large focal length range, but it only zooms out to 28mm (42mm equivalent), which is not very wide on a crop-sensored camera like your D3100. The 28-300mm is really designed for full frame use where it gives about the same coverage as an 18-200mm lens would on your D3100. This may be a better choice if you want an all-in-one lens. But it all depends on your style of shooting. 42mm may be wide enough for you. You could of course combine it with a wider angle lens. Or go for a two-lens setup with the 70-300mm covering the telephoto end
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Canon DSLRs: 5D Mark III, 7D
Canon lenses: 17-40/4
L, 24-105/4
L, 24/1.4
L II, 50/1.4, 85/1.8, 100/2.8
L Macro, 70-200/2.8
L IS, 100-400
LOther stuff: Canon 580 EX II, Canon 430EX II, Velbon Sherpa 600R
Apple: MacBook Pro 15", iMac 27", iPad III, iPhone 4S, Apple TV
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