I'm considering the less expensive Sigma 18-300 DC Macro lens vs. the (way) more expensive Nikkor 18-300. Would I be losing the gains by buying a 24 MP Nikon 3200 if I went with the Sigma?
You'd be sacrificing a significant portion of the capabilities of the D3200 by using either on of those lenses on it. Zoom lenses with a wide range of focal lengths force the designers to make many compromises along the way. The best quality for the lowest price is usually found in prime lenses (lenses with only one focal length). Some of the best and most expensive zoom lenses that cover about a 3x focal length range (such as the 24-70 f/2.8 or 70-200 f/2.8 pro grade lenses from various manufacturers) cost about 5-8 times the price of primes that yield comparable image quality. You won't be able to find an 18-300 that gives wide apertures and the same image quality at any price.