In multitasking and multithreaded applications i3 will be better due to it being a four threads cpu vs. G630 bei9ng a two threads CPU.
Here is G630 vs. i3-2120http://www.cpu-world.com/Compare/242/Intel_Core_i3_i3-2120_vs_Intel_Pentium_Dual-Core_G630.html
Just click performance tab.
Whether it is worth it is up to you. If you are pretty heavy into it or just enjoy higher performance, get i3. If you want to save some money and a little bit less extra time doesn't matter to you, get G630.
Note that in some games G630 may struggle a bit more such as BF3 Multiplayer (Not single player).