I've shot with a $40,000 Hasselblad H4D-60 - back when they were new. That's a 60 megapixel camera. I've shot the Canon 1Dx at 18MP and the Canon 5D MKII at 21MP. My main still camera is a Nikon D4s at 16MP currently - and by far - of these 4 cameras - the best images have come out of the Nikon D4s.
Megapixels equate to detail in most cases... but LATITUDE is what makes one camera superior over another. I have a Bachelor's degree in Film Production - specifically cinematography. I learned on film. And really, the difference between a photographer and cinematographer is how many frames per second we capture.
I've not been a fan of digital motion picture cameras until just recently because they lacked one key thing that film just does better... that's dynamic latitude. In cinematography shutter speed is a constant. It's 24 frames per second. So exposure for us has to be adjusted through the ISO of the stock we're shooting with or aperture. Think about putting your digital still camera's shutter at 1/48th of a second and never changing it again. That's what I learned to deal with. LOL. If was frustrating - still can be at times.
A stock's latitude is measured in stops. And regardless of whatever fancy stuff they are going to do to my footage in post production - I always aim for the best possible "normal exposure". Meaning detail across the entire exposure - from the darkest areas of the frame to the lightest. Kodak Vision 3 cinema stock has 15 stops of latitude. I can film someone in shade indoors next to a window and still get color and detail through the window. Ironically, most lenses only have 8 to 10 f-stops.
What makes film look so rich and so beautiful? It's the latitude. The ability to capture realistic, saturated (but not over or under) colors and tones very similar to what our eyes see. Digital has always lagged behind. If I was to film the same scene mentioned above - a person in the shade indoors next to a window - with a top-of-the-line digital motion picture camera you'd not see any color information or detail through the window. It'd just be white.
The problem with digital cameras at first was the size of the sensors. They were smaller than a 35mm negative and therefore they presented more focus - deeper fields of focus - and less ability to capture fine detail. Then there was latitude - the Red One - the first motion picture camera with a 35mm sized sensor - has between 5-7 stops of latitude and captures 4:2:2 - it compresses colors and detail. Digital still cameras have worked the same way.
So for me the Megapixel Myth is about the size of the sensors - The thing to be very wary of is not the physical size of the sensor (how many mm this way and mm that way) but how many pixels are packed into the area of the sensor. Nikon and Canon had both taken the full-format top-end cameras they produced and had started packing smaller and smaller pixels into the sensors. So the MP count went much higher. They got to around 24MP and noticed that their mid-range cameras in the 14-18MP range produced better images.
That's the myth - more MP in a sensor - meaning smaller-tigher pixels - doesn't increase image quality - it actually decreased the quality. Larger pixels have more dynamic range. They produce lower noise levels and they can capture more photons. If you compare photos of the same subject shot by my Canon 5D MKII and my Nikon D4s - even though they have the same size sensor (physically) - the Canon has more pixels packed into its sensor - (it has 5 megapixels more) - hands down the Nikon shoots richer, clearer, sharper, more striking images.
For me - I don't care about megapixels. I think that the image quality is better served by providing less (or ideally zero) color compression and more dynamic range/latitude. I'm more happy with my 16MP photos from the D4s than the 21MP photos from my Canon 5D MKII.
Truthfully, when asked which digital camera I prefer in my Monday-Friday work as a cinematographer - it's simple - none of them. Give me a Panavision or ARRI 35mm any day of the week.