3G and 3.5G are used for old Universal Mobile Telecommunications Systems.
3G is UMTS and 3.5G is UMTS with High-speed packet access (HSPA or HSPA+).
3G(UMTS) data rate up to 2Mbits/sec but 3.5G(HSDPA) UP To 14.4MBits/secs.
The main Tech. difference it that HSDPA uses AMC scheduler.