Which one do you suggest me to go with? 1. Sigma - APO 70-300mm F4-5.6 DG MACRO for Nikon 2. Tamron - AF70-300mm F/4-5.6 Di LD Macro I would go to National parks and reserve forests as well for shooting which have low light in some cases. I have a nikon D50 in excellent condition. My budget is about $300 and I can't stretch it any more. Please drop your suggestions (suggestions about other brands are also welcome) Thank you in advance, Ravi.