The difference in advertised contrast for those two screens is not really worth considering, as they both use TN panels, which tend to have contrast ratios of around 1000 to 1. "Dynamic" contrast ratios that are often advertised with values in the millions are complete marketing nonsense. If there happens to be any actual difference in contrast between those screens, you won't find it in the advertised numbers, and would have to check reviews for that information.
From the look of it, the G2590FX is a slightly larger screen at 24.5", and has thinner bezels surrounding it.
The G2460PF seems to offer a more adjustable stand though, with height, pivot, swivel and tilt adjustments, while the G2590FX only offers tilt, and the G2460PF also has a USB hub. There is apparently a "PX" version of the G2590 that also includes the better stand and the USB hub though, if those were things you were interested in.
I did find some thorough reviews for these two screens, but only quickly skimmed over them...
A review of the AOC G2460PF, a 144Hz gaming monitor supporting AMD FreeSync variable refresh rate technology.
pcmonitors.info
A review of the AOC G2590PX. A 24.5" Full HD TN model with 144Hz refresh rate, slim bezels and support for AMD FreeSync.
pcmonitors.info
Note that the G2590 review is for the PX version, which has the better stand and USB hub, but it sounds like the FX version should otherwise be similar.