If your pc renders 120fps then there is no performance increase by doing 120hz monitor.
The benefit is on your eyes by providing a cleaner and there for sharper image so it means your more likely to notice things during fast pace.
The performance really comes from the frame rendering as thats when the data is generated, the hertz is essentially how many times your monitor is being told to clean the screen completely.
Imagine you have 400 sticky notes that have an image drawn on them that when you flick through them fast they move and the faster you go the smoother the image looks.
The drawn image is the data and the "flick" of each note is the refresh rate of your monitor. The data is already there and waiting to be displayed but the faster you flick through the notes the clearer your eyes see it.
The monitor is similar.
In a strange way there is a performance increase but thats for your eyes to see it so the performance increase matters within yourself
I will also tell people it depends on your budget. Get the framerate over the hertz on a monitor but if money isnt an issue then get both!
Hope this makes sense, typing it from my phone at work haha.
- Ryan
