Generally speaking, the difference between a dual core i5 and a dual core i7 CPU is the clockspeed. There are some other small technical differences like more internal cache, but that is more or less unimportant for the average user and gamer.
When it comes to 4th gen vs. 5th gen... 5th gen CPUs consumes a little less power which means potentially longer battery life. There has been a slight improvement in performance (about 4% on average) so a 5th gen CPU at 2.7GHz would be slightly more powerful than a 4th gen CPU at 2.7GHz. In other words, a 5th gen CPU at 2.7GHz would be about equal to a 4th gen CPU at 2.8GHz. The biggest difference is in graphics performance; the newer Intel HD 5500 graphics core is roughly 15% - 20% more powerful than the older Intel HD 4400. But the actual performance difference depends on the specific game.