A megapixel (MP) is 1 million pixels. The more megapixels you have, the more image data you can capture. None of the cameras here have less than 10MP. If an image was captured well (in focus and exposed properly), you could print it out confidently up to 40" and beyond with any of these camera options.
All of these cameras have more than enough megapixels. Really.
Now that the playing field is somewhat even, it is important to consider the quality of sensors, ISO attributes, and the quality of your lens.