Iv'e just said on another thread that I was advised by Puddleduck not to spend extra money on the ED version of the Nikon 70-300 as opposed to the standard 70-300 which seems to be available for about £100. I can't comment on the Sigma except to say a friend has one and seems well pleased with it, albeit that he bought it because it was cheaper than the Nikon equivalent.
I'd have to agree with flash in the pan though, if you can stretch to a VR version you should go for it, although it's probably a bit to early to get a used 55-300.