Honestly those are the same CPU. The 4590 is a few percent better than the 4460, which is the older 6400, which is better by a few percent. So 4590 is a 6400. And a 6500 is a 4690. And a 6600 would be a stock 4690k.
So I mean if you think a few perce it's worth it, go 6500 for 20 more than the 6400. Skylake because it's more efficient