One of the difference between the i3 and i5, is the i5 has "turbo boost". The i5 seems to be limited to a 266Mhz 'turbo-boost'. If you do opt for an i5, do pay attention as not all of them have the integrated graphics. Here's some more info from Anandtech : http://www.anandtech.com/show/2901
From Wikipedia:
Intel Turbo Boost is a technology implemented by Intel in certain of their Nehalem-based CPUs, including Core i5 and Core i7. Turbo Boost allows CPU clock speed to be dynamically increased on demand. It is activated when the operating system requests the highest performance state of the processor.
When the processor is operating below its thermal and electrical limits and the user's workload demands additional performance, the processor clock frequency will dynamically increase in increments of 133 MHz on short and regular intervals until a thermal or power limit is reached or the maximum speed for the number of active cores is reached. Conversely, when any of the limits are reached or exceeded, the processor frequency will automatically decrease in increments of 133 MHz until the processor is again operating within its limits.