300dpi is good for sharing photos with family and friends. It also takes up less space and is easier to share cause the file size is smaller. If you scan a 4×6 at 300dpi you can make a enlargement no bigger than a 5×7.
600dpi you get more resolution, so the image will not pixelate if you make a 4×6 into as large as a 11×14. Also smaller images like a 2.5×3.5 will benefit from the higher resolution scan when you view them on a high resolution screen like a 4K tv.