GIMONO

Technically DDR3 ram is advance than DDR2 ram, due its core clock  and high speed, in market the prices of it are cheaper when compared to DDR2 ram

The difference come in clock speed as DDR3 runs at higher clocks and lower voltages due to a more complex 90nm manufacturing process while DDR2 heats up fast. Though in order to change DDR2 ram to DDR3 ram, one must change his motherboard  to support DDR3 ram,
its a bit costly, so its  preferable to us DDR2.

you can the difference in system rating if  you are using a DDR3 ram, it will improve the average score of system. 

Windows 7 ultimate system rating : ram : 7.1 when using DDR3 ram
Windows 7 ultimate system rating : ram : 6.9 when using DDR2 ram

For more info click on this link

So, there isnt  much difference.