While the i5 is only a dual core CPU, the Y510p that comes with it has dual GT 755m cards while the quad core i7 version has only one GT 775m. If gaming is the ultimate goal, then get the i5 version because there have been quite a few benchmarks that have shown a laptop with a stronger GPU and weaker CPU outperforms a laptop with a weaker GPU and stronger CPU. Keep in mind that the 2nd GT 755m will only be used if the game supports SLI.
Personally, I prefer a laptop with a single graphics chip / card since all games are capable of using at least one GPU. There's also less noise and less heat.