The i3 being a dual core and the i5 being a quad core is itself a major difference. The extent of difference can be seen in the benchmarks. The cache count is mathematically same i.e. per core cache is same on both (256KB L2 and 1.5MB L3). Other than that, the TDP of the i3 2100 is 65W as opposed to 95W on the i5 2500. The i3 2100 also has no turbo boost feature.