I want to upgrade my Nikon 24-70 2.8 ED to another 24-70 with VR. I will also be picking up the Nikon 105 mm 1.4 or the Nikon 70-200 ED VR 2. My question is, have any of you compared the Nikon duo vs Tamron's G2's? Is the few thousand bucks worth it to go with the Nikon lenses? I am assuming the biggest difference is the focusing system in low light. I just want to make sure I am not picking up the first party lenses over the third party just because of the name.