Nikon 70-300VR or Sigma 50-500 suitable for Safari?

Hi, first time posting, but I must confess to having learned quite a bit from these forums (to the point of recently trading in my trusty D70 for a D300s).

Anyway, thank-yous aside, I was hoping for some advice from you: I am planning a holiday to Kenya and Tanzania with a few game drives here and there, and I wondered if the nikon 70-300 VR that I have should be swapped for the Sigma 50-500 for added range or whether I should just work with what I've got? Is there a trade-off here for image quality vs range?
Many thanks,
RichardP