I remember when I got my first camera I was so excited because it had TEN megapixels. I (like many others) was under the impression that more megapixels meant a better image. While that CAN be the case, it really just depends on what you are using the image for.
Most of the time, less megapixels means BETTER low light performance. Think about a pixel as a bucket and think about light like rain. The bigger the bucket, the more light it can collect. Given the same area, an image sensor with LESS pixels captures MORE light per pixel.

This is the reason cell phones typically do not do well in low light. They may have 8 or 12 megapixels but the pixels are TINY because they are all crammed in that tiny little sensor.

Besides having a shallow depth of field (we will cover that in a future video) full frame cameras perform better than smaller crop cameras because their image sensor is bigger, meaning their pixels can be bigger and collect more light.  

If you are shooting for a GIANT display (think 8′ billboard) then you may want more megapixels. The more megapixels, means the bigger the image can be. If you are shooting for fun or to publish primarily online, a 12 megapixel camera is plenty!