Hi Mike, or Nel
The Nikon lens is amongst the best optics in the world, imo, and that 35mm is equivalent to 52.5mm for 35mm film. TBH, it's a lens length I rarely used, because it mimics the angle of view of the human eye, and part of my photography is about bringing a different perspective to the picture. That said, the extra 1.6 stops will come in handy in low light conditions, though it has no VR on it.
The Sigma is a good lens, and a 24 is equivalent to a 36mm lens. To me, that's a middle ground lens, not wide enough to be really wide angle, so of the two, I'd probably buy the Nikon.
My own current range is the 14-24 (21-36) f2.8 zoom (weighs a kilo, but it's a great lens), 24-70 (36-105) f2.8 zoom, an 18-200 (27-300) 'single lens' zoom, f3-5-5.6, and a 50mm (75 equivalent) f1.4 prime. I'd normally be looking at something nearer 60 for this, but they don't have one at present. This lens is great for portraiture and low-light shooting. I'm planning to add the 60mm macro (or micro as Nikon call it) when the VRII version hits the streets, and I'd like a wide angle prime 24-28mm.
However, the real answer to your question has to start with a question, what sort of pictures do you want to take, ie subjects?
