I5 has turbo boost whereas I3 doesn't have Turbo Boost. That makes considerable difference in Single threaded applications.
However , if you were to use only the iGPU , then ofc its the later.
In Short : If you are planning to buy a laptop which doesn't have a descrete/dedicated GPU then you should opt for Ivy Bridge processor(Core I3) over the Sandy Bridge(Core I5).
Otherwise , if your laptop has dedicated graphic chip , then Core I5 is what you should get.