The i5-4590 is slightly better in performance and since they're the same price, I would purchase the 4590. However if you're using an 8 series motherboard (H
87 for example), you may need to update your BIOS to support the i5-4590 as it is a Haswell Refresh CPU.
If you're on a 9 series motherboard, you may simply purchase the i5-4590 and it'll be compatible out of the box. If you already have a processor with your current motherboard, I'd update the BIOS first and then order the i5-4590. Otherwise without a BIOS update the CPU may not be compatible, whereas the i5-4570 will be compatible as it's an older CPU.
Again it depends on your motherboard and whether it will accept the newer Haswell Refresh CPUs with or without a BIOS update.
All the best.