Hello aturi - choose the aspect ratio that seems most suitable to the way you see the image and what you would like to see in it. For example, a 16:9 aspect ratio is usually suitable for landscapes, as we usually like to get a wide view of them. In other circumstances, a ratio of 4:3 might be more suitable to how your eyes naturally view an image. A ratio of 1:1 has a certain beauty that can make a composition more harmonic. But any of these ratios can be used in interesting ways - all depends on what you're trying to convey in the image.
The difference in pixels is not significant to the quality of the image taken - just a number that says (in the case of a lower count) that part of the pixels of the sensor have not been used, because of the way the image has been framed. Think of it as removing a circle from a square - some of the square will be left behind, but you still have a valid circle.
Hope this is what you were asking.