It really just depends on how professional you want to make your pictures. As for your first question, the jpeg is great for sharing the picture on your phone and smaller screens. the 16 mega pixel .DNG file is for editing in Light-room or Photo Shop. The point of having a 20MP sensor is what you can do with photos post production. With an image that large, you can zoom in on the image and still retain a lot of detail the closer you get. as far as aspect ratio, shooting in 4:3 is best for still images that are average photo size, more square shaped. 16:9 is more popular with images you are blowing up to bigger size or movies as most movies are now shot in widescreen format.