The main difference between the two without getting too complicated and technical. Is that the 900Mhz has a slower bandwidth per second than the 2100Mhz.
However , the 900Mhz frequency range can travel a lot further and provide a wider range. The 2100Mhz frequency range is shorter but a lot denser, thus you will be able to transfer data at a higher rate with 2100Mhz than you would with a 900Mhz frequency range.
Hope this has helped you.