I agree the VR feature is a big plus especially the bugs you are trying to shoot in the back yard won't wait for you to set up a tripod, that's when the fountionality of VR becomes necessary.
Unlike reef equipments, camera lenses usually hold it's value, by all mean get the Sigma and when you save enough dough you can always sell the lens for an upgrade. For example I got the Nikkor 105mm VR for $800, should be able to auction it off on Ebay for $700-750 easily.