i5-4690 is a little better than i5-4590 but the difference will be marginal. Neither will bottleneck the GTX 970.
i5-4690 is $215 right now. i5-4590 is much cheaper at the moment giving marginally less performance making it a better buy at this moment in time. Make sure your BIOS will support the CPU before upgrading. Otherwise, update your BIOS.
Good luck