The i3 -> i5 difference is a noticeable one when it comes to an application that's more CPU dependent.
Between the 4670k and the 4590. There is little difference between the two if you're not going to overclock. If you get the 4670k, even if you don't think you'll overclock, you might do it later on, so there's that option. It's not very hard to do either, just a bit time consuming.
But answering your question. The difference between the two i5 cpus is only the frequency I believe. There performance should be relatively the same. If you really don't plan on overclocking, then going with whatever's cheaper would be best.