I am looking to buy my first macro lens and I am between two: SIGMA 70-300 mm f/4-5.6 DG or Tamron 70-300mm F4/5.6 DI LD Macro (Nikon AF). Which one is better ?
Neither of those lenses are really what most macro specialists would consider a macro lens. For a lens to be considered a true macro lens it should be able to project a life sized image of the subject onto the image sensor or film. If you're taking a picture of a 20mm long bug, a macro lens should be able to focus close enough to project an image of the bug that is 20mm long. This referred to as a 1:1 reproduction ratio or 1.0X Maximum Magnification (MM). There are a few groups who define anything with an MM of more than 0.5 to be a macro lens, especially if that lens is designed to be used with a camera that has a smaller sensor or uses a film format smaller than the standard 36x24mm full frame/135 film format.
The Sigma has an MM of 1:4 or 0.25X.
The most recently introduced Tamron 70-300mm f/4-5.6 (A005) has an MM of 1:3.9 or 0.26X. It's more expensive but also gives better image quality.
The older model (A17) listed in the question has an MM of 1:2 or 0.5X
So in terms of macro capability there's little difference at all between the two currently manufactured lenses. Neither really comes close to being a real macro lens. The older A17 it does have a better reproduction ratio, but is is still short of what many consider to be a true macro lens.
In terms of other optical performance there's also not a lot of difference between the two. They are very similar in their optical design and very similar in their performance.
Perhaps the biggest difference between the two is in ergonomics. Tamron lenses tend to zoom and focus by turning the rings the "wrong way" like Nikon lenses do. (I'm kidding. Sort of.) As viewed from the camera you turn the focus ring counterclockwise to focus on near objects and clockwise to focus on far objects. You turn the zoom ring counterclockwise to zoom to a wider focal length and clockwise to zoom to a longer focal length. Sigma lenses tend to zoom and focus by turning the rings in the opposite and "correct" direction like Canon lenses do.
the Sigma 70-300 is my opinion is: 1. loud during auto focus 2. often cannot find focal point 3. if you carry your camera in a side sling as I do gravity will extend it out to 300mm it does not stay where you left it. That said this lens works best outdoors I have great fun at the zoo with it. You can gets some almost Macro shots but it is a challenge to come out of Macro mode it is not just a flick of the switch. Also, if you are trying not to spook wildlife it is not the lens for you. The Tamron is quieter and if you must have this size lens it is the better option in my opinion.