WTB: Sigma 120-400mm or 50-500mm for Wildlife Photography

Hi, I am looking to buy a second-hand super telephoto lens pretty soon for wildlife photography and am keen to hear thoughts from anyone who has used the Sigma 120-400mm and/or Sigma 50-500mm. I have had the chance to try using the 50-500mm briefly... which seemed rather hefty to cart around.

If anyone is considering selling one of these two lenses in the near future, that would be great to know too.


Thanks,

Phil