Someone asked me why anyone would spend $6,000 on a 16 megapixel Nikon D4 when they could get the 36 megapixel Nikon D800 for only $3,000.
With digital cameras, there are three things that are important: the sensor, the software, and the lenses. The D4 and D800 both have Nikon's excellent Expeed 3 software built-in. The D4 and D800 have the exact same lenses available. So the only important difference is the sensor.
According to DxOMark, the D800 has the best sensor of any digital camera, while the D4 has the third best. Both sensors are excellent, but the D800 has the better sensor.
So why would anyone spend $6,000 when $3,000 buys the better camera?
The only reason to buy the D4 instead of the D800 is if you are an action or sports photographer. The D4 is quicker than the D800. It's not just the 10 frames-per-second of the D4 vs the four frames-per-second of the D800, but it's also the write speeds, memory buffer, and ability to auto-focus and auto-expose correctly quickly. The D4 is microseconds faster than the D800, and those microseconds could make all of the difference to getting "the shot" or not.
For the vast majority of photographers, the D800 is the better camera (and for half the price!). For some, the D800 is just not quite fast enough and you'll pay $3,000 more for just a little extra speed (ouch!).
Just one more thing to think about: the Nikon D3200. It also has Expeed 3 and the exact same lenses available. The only difference is the sensor, and the D3200's sensor is actually pretty good. For $2,300 less than the D800, that's a pretty darn good deal.
Actually, it doesn't matter which camera you buy, because your camera matters not.