Is getting a Z chip set and a locked multiplier cpu the best use of your $$s? You probably could use a B or H series chipset for equal performance and less cost. Or you want to step up to the K model i5 to take advantage of the Z chipset. just my 2 cents though.
To your question 4590 at same price is better than 4570, but I wouldn't pay more if the 4570 is >$20 cheaper.