Refresh rate (Hz) is how often the display panel redraws the screen each second. Basically equivalent to frames per second (fps).
A 120Hz display would only be "better" than a 60Hz display if your GPU can reach 120fps (or at least sustain >60fps) at the display resolution.
A Mobile GTX1070 (which is slightly better than a Desktop GTX1070) just can't sustain 120fps on all games at "FHD" 1920x1080, and it can't sustain 60fps on all games at "4K UHD" 3840x2160, even with quality settings turned down.
If you plan to also/eventually use the monitor with other computers (and better GPUs) then I'd recommend you get higher resolution.
If you plan to just use the monitor with your GL502 then I'd recommend getting the cheaper one.
I'd also second Nate's recommendation of a 2560x1440 60Hz monitor, it would balance pixels and speeds and your GPU pretty nicely.
I didn't watch Nate's entire video. But I did see "somewhat Ultra Quality" 2560x1440 Watchdogs 2 performance generally hover around 30fps~60fps.
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