Since 4:3 is the native resolution of the sensor, that ratio uses full capacity of the camera sensor, whereas 16:9 only uses 75% of the sensor. If you take the picture in 4:3 you can always crop down to 16:9 and still have the same resolution you would have had if you'd use 16:9 in the first place. But you can't go the other way, without losing quality (resolution).
I think the general consensus among photographers is that 4:3 is a better general purpose aspect ratio, which is why nearly all smartphone and higher end cameras have a 4:3 native resolution.