I have a nikon d5100 with kit lens 18-55mm..am planning to purchase a new telezoom lens for it. my budjet is very low (maximum 10k)..am considering sigma and tamron 70-300mm..but am confused between these two..TAMRON AF 70-300mm F/4-5.6 Di LD & Sigma 70 - 300mm F4-5.6 DG Macro...(with built in motor)..i knw sigma have another version of this lens with APO ( Sigma 70 - 300mm F4-5.6 APO DG Macro) but its around 15k..so which is better sigma or tamron?and wat's APO?is it really matters?