They are, in essence, the same phone. They are both Curves that come with internal GPS. The Curve 8330 is the CDMA version of the Curve, which runs on networks like Sprint, Verizon and Alltel. The Curve 8310 is for GSM networks like AT&T.

8310 does not have video recording.
8330 has a better hearing impared rating.
8310 has 17 days standby and 240 minute talk time and 8330 has 11 days standby and 260 minute talk time
8330 has 96 mb of flash memory and 8310 has 64 mb.
8330 is on Dual-band 800/1900 Mhz CDMA2000 1X Ev-DO networks whereas 8310 is Europe/Asia Pacific: 900MHz GSM/GPRS networks, Europe/Asia Pacific: 1800MHz GSM/GPRS networks, and EDGE networks

the Blackberry website lets you compare the two phones and their features.