Tamron or Sigma lens?

I want a new lens for my Nikon D5100 and don't want to spend all that money on a Nikon lens. I want a 70-300mm lens but don't know whether to choose Sigma or Tamron. Which one would you prefer and which 70-300mm lens from that brand would you get?

Are you aware that Nikon has a 55-200mm lens you can get new for about $230? Or refurbished for about half that price?
http://www.kenrockwell.com/...0mm-vr.htm

It more perfectly matches your 18-55mm kit lens for optical lengths and is superior optically.