Intel’s New i3/i5 Chip Families

Code named Westmere, the Core i3 and Core i5 6xx processors are built on top of Intel’s Nehalem architecture. Westmere’s built on the newest 32nm manufacturer process with the expected result in lower power consumption and this is indeed true. Also built into the new chip are SSE instructions aimed at accelerating encryption and decryption with the AES algorithm, a benefit for those that use data and drive encryption to protect their mobile systems.
Both chips support hyper-threading, the Core i3 processor does not support Turbo Boost which means that they do not automatically clock themselves higher when the workloads allow it. The low-end Core i5 processors can only overclock themselves by a maximum of 266Mhz with their Turbo functionality so this is not as big a difference as you might have thought at first.
The most exciting new addition to these processors though is the addition of an integrated graphics processor on the same die is the CPU. They are still separate entities, but they are kept on the same piece of hardware.
In summary, what this really means is that the Core i3 is the processor of choice if you're on a budget. It only loses Turbo Boost, which in the low-end Core i5 processors is not an impressive feature. The Core i5 processors are faster, however, so if you're not constrained by budget they are also a good choice.
If you are into numbers… Michael J. Miller from PCMAG.COM compared two units from Lenovo, one an i3-330 2.13GHz Core against a 2.2GHz Core 2 Duo T6600. The prices are very similar so this was a great place to start. In the graph below the lower the number the better. The dramatic differences were seen when using graphics applications and applications that could make good use of hyper-threading.