Nikon SB-600 or Sigma EF 500 Speedlight questions

[blazer]

Looking for some advice on which speedlight to buy. I have it narrowed down to the nikon sb-600 or the sigma ef 500, due the fact that I have a gift certificate to a store that sells these speedlights. I'm wondering if there is a major difference between these speedlights and will both of them work on a D-50? Thanks in advance for your responses.

 

[JIP]

Save the gift card for something else go for the SB-600 for that matter go for the 800