Question about a lens

[kinghen]

I will be purchasing a lens soon. I would like to buy a Nikon or Sigma 50mm or 105mm. I have a D3100 and would like to know which brand would be better? I know the price will be different but not sure about the brands. Any help would be appreciated.
Thanks, Henry

 

[GRbenji]

You need to be aware that your D3100 doesn't has a inbuild AF motor and needs lenses that have inbuild AF motor to do autofocusing. Both the sigma 50 and 105mm macro lenses do not come with inbuild AF motor. So buy them only if you planned to use solely manual focusing.

As for differences between 50 and 105mm, you need to know the pro and cons of longer and shorter focal length. Longer focal length gives longer MFD at 1:1, good for skittish subject. Longer focal length gives smaller FOV, hence cleaner background. Shorter focal length less affected by handshake, hence better for handheld (105mm is still good for handheld shooting).