Various Types of Lenses: Nikon Vs. Sigma
Normal Lenses: A normal lens is one with the same range of vision as the human eye. It is equivalent to a 50mm lens for a 35mm (full format) camera. On a smaller sensor camera, the equivalent lens is a 30mm which is available in Sigma, but is not manufactured by Nikon. This makes the Sigma 30mm f/1.4 EX DC a very good buy.
Read 3 Reasons You Should Own a 50mm Prime Lens to understand why a 50mm prime lens (normal lens) is essential for every photographer.
Wide to Telephoto Zoom Lenses - Standard "Kit" Lenses
These lenses (18-55mm/18-50mm) are usually supplied as Kit lenses and have one major weakness - an extremely small aperture (e.g. f/4 or f/5.6) which makes it difficult to shoot in low light without a tripod. The pro lenses from both Sigma and Nikon are f/2.8 which solves this problem. The Sigma 18-50mm F2.8 EX DC is reasonably priced. However, in the absence of price constraints, I would go for the 17-55mm F2.8 lens from Nikon which is of better optical quality.
When it comes to the 50-150mm, 70-200mm or the 70-300mm zoom lenses, I would go with the Nikon lenses as there have been many instances of complaints related to these Sigma lenses.
Read The Best Nikon Zoom Lenses for Your Digital SLR to learn more.
The macro lenses from Nikon and Sigma are available both as prime lenses with shorter focal lengths and as telephoto lenses with longer focal lengths. The shorter ones are cheaper and the Nikon lenses are preferable due to superior optical quality. However when it comes to the longer focal lengths, the Sigma lenses offer extremely good quality for their price point.