Shawn
This is a really hard question to answer and you will get almost as many different points of view as there are photographers.
It is true what Vincent says, however, my advice is to concentrate on the image making side of things. Keep things simple, get used to the gear you have, and only then think about “up gradesâ€. Having fine optics does not make you a better photographer. It is not so much about the gear you have but what you achieve with it.
The problem with the Nikon range is that it offers too much choice. Both Sigma and Nikon make high quality optics, but you need to do some hard research to find out the best performers. At the end of the day it comes down to your own preferences and requirements.
Most importantly enjoy your photography.
Cheers and good luck, Craig