HSDPA is faster than 3G.
3G (3rd Generation of Telecommunication Standards) is a standard of cellular connectivity which allows for speeds of up to 384 Kb/s downstream and 64 Kb/s upstream. 3G HSDPA is a 3G enhancement. HSDPA (High-Speed Downlink Packet Access) is a new mobile data protocol and is sometimes referred to as a 3.5G. Theoretically, HSDPA speeds can reach 3.6 Mbps downstream and 348 Kbps upstream. But the purpose of putting both on a smartphones is to make sure that your phone get internet access even when the other one is not available or have no access