Both are good. And both will probably work fine for whatever application you have in mind. But the i5 520m is faster at 2.4GHz and turbo boost will bring it up to 2.93GHz for single threaded applications. The i3 is at 2.133GHz. The i5 will have significant performance gains. Whether you notice those gains or not will depend on the applications you use and what you intend to do with the laptop.