Many years ago we had a 6 hour computation and did some trial runs with what some thought would make a big difference. Result? About 10 minute advantage over the 6 hour run.
HOWEVER forget the 3rd gen unless it's at a significant discount by hundreds of bucks. No sale here unless there's a discount.
In closing you seem to be falling for the MegaHertz or GigaHertz MYTH. Yes, it's a myth. Do you need links?
They are both asus laptops, with majority of the specs being same. Except that one is i5 4th gen with 1.6ghz processor and turboboost to 2.6ghz. The other is an i5 3rd gen with 2.7ghz of CPU power. Both come at about the same price.
So i really m confused in deciding the right one.