1) USB 3.0 is claimed to be 10 times faster than USB 2.0 as USB 3.0 can transfer data at speed up to 4.8 Gb whereas maximum data transfer speed of USB 2.0 is only 480 mbps only.
2) With USB 3.0, you can write in both directions simultaneously whereas with USB 2.0, you can write in one direction only. (You can read and write simultaneously with USB 3.0)
3) USB 3.0 consumes much less energy as compared to USB 2.0 thus increases battery life of your device.
USB 3.0 is great discovery for all computer users because it will decrease time spend on data transfer considerably and make external devices faster and less energy consuming. So far, USB 3.0 based devices are slightly more expensive than USB 2.0 based devices and they are available in less numbers. However, with time we can see decrease in prices of USB 3.0 based devices and increase in availability of USB 3.0 based products.
Both USB 3.0 and USB 2.0 work on each other; however, you will get maximum speed only if both devices are USB 3.0 compatible.