The two Core i3 CPUs are 5th generation Broadwell CPUs. The difference is probably 100MHz or 200MHz when it comes to the processor speed. Also, Core i3 CPU lacks Turbo Boost which is basically an automatic overclock when the CPU is being stressed as long as it is not running too hot. Without Turbo Boost Core i3 have lower clock speeds compared to Core i5 CPUs. They have the Intel HD 5500 graphics core.
The Core i5-4520 does not exist so I am going to assume you means the i5-4250u. It is a 4th generation Haswell CPU. There is only about a 5% difference in overall performance between 4th and 5th generation CPUs assuming they run at the same clock speed. The i5-4250u can hit higher clock speeds than the i3-5010u CPU and therefore, it will be more power. It has the Intel HD 4400 graphics core.
The Intel HD 5500 is generally around 15% more powerful than the older Intel HD 4400, but actual performance varies from game to game. Since most games rely more on the GPU than the CPU, I suppose I would go with the i3-5010u with the Intel HD 5500.
The Intel HD 5500 is not very powerful compared to dedicated GPUs so performance in many games will generally not be very good unless you lower the resolution and graphics quality which may give you acceptable performance.
Below is a link to more info about the Intel HD 5500 and some game benchmarks. Note that the laptop used for benchmarking usually has a Core i5 CPU and in a few cases a Core i7 CPU. It does pretty well in FIFA 14, and can do decently well with FIFA 15 & 16. However, it struggles with BF4. It does poorly with NFS: Rivals and of the 3 different CoD games listed it only does well in CoD: BO2
http/www.notebookcheck.net/Intel-HD-Graphics-5500.125586.0.html