DDR3 performs faster, and consumes less power as it works and cheaper price than DDR2 for now , also can say Old & New
Performance Differences :
DDR3 uses significantly less power, and can run much faster. It uses 1.5V per chip, which is 0.3V less than DDR2. Additionally, when it comes to its Data rate, DDR2 (depending on its chip) can run at 400-1066 MT/s (megatransfer per second) and and some mobo support to 1333. In regards to its peak transfer rate, it can transfer from 3200-8533 MB/s. DDR3, on the other hand, can run from 800-2133 MT/s and 6400-17066 MB/s. In regards to the I/O bus clock, DDR2 is only available between 200-800 MHz, whereas DDR3 supports from 400-1600 MHz. Essentially, DDR3, when there are improvements from DDR2, can double in processing power. The fact that DDR3 consumes less power in outperforming DDR2 is impressive.