Knowledgable photographers know that there are many factors that affect image quality, and that probably the most over-rated and over-hyped is "megapixels." It's an easy comparison point to include in camera marketing, but it rarely by itself tells you what you need to know. First of all, the number of pixels only influences sharpness, and there are many other factors that contribute to image quality. And even that one factor of sharpness is influence by many sub-factors--lens quality, camera alignment, in-camera processing, and strength of the anti-aliasing filter, to name just a few.
So does that mean that the number of pixels is irrelevant. No, it doesn't. With a high quality lens, a steady photographer, and a well-aligned camera in perfect focus, significantly more pixels make a sharper large print.
Here are two crops. The Nikon D300 crop is 1:1 from an excellent 12 megapixel camera. The D5100 crop is from a 16 megapixel camera. Both were shot under identical conditions, on a tripod, using a self timer, with an excellent 50mm f1.8 prime lens at its sharpest aperture.
As you can see. in this comparison, under ideal conditions, the D5100 image is noticeably sharper. My guess is that most photos are not shot under idea conditions, probably not on a tripod, and probably not with a super-sharp lens. And, most people don't print their images very large, if they print them at all. Bottom line, for most photographers, a 25% difference in the number of pixels won't make much of a difference in most situations.
For me, it does make a difference. When you shoot professionally, you never know how or where your photos will eventually be used, so I try to make every image as sharp as possible. (I've had some pictures I shot for use as a 4x6 inch picture in a publication be later purchased for making 4x6 foot mural prints. Visit the new Coast Hills Credit Union office on Marsh Street in San Luis Obispo to see how some of them look.)