Generally speaking the i5 will perform better than an i3 because Turbo Boost allows an i5 to automatically overclock itself above the stock speed based on the number of cores being used and the CPU's temp; at most 400MHz, at least 200MHz. Core i3 CPUs lack Turbo Boost.
The i5 is also a quad core CPU, but not all codecs can use 4 cores when encoding / editing video. For example, the XviD codec really only uses one CPU core, not two cores as the developers claims. However, the H.264 codec can make use of 4 cores.