1440p or 1080p for new gen

It's personal preference

Just as a word of advice I'd verify in advance your monitor is known to work with Xbox. Monitors that are overclocked to non-standard rates like 165hz often fail to work, especially at higher rates because the hardware and software driving those panels can't support it.

I don't really see much of a difference, but you might. Try both modes, and whichever one you believe is better for you, then use.

How do you mean? It's a 165hz monitor can be overclocked to 180hz but to run that you need a pc even to get 165hz due to the consoles cap at the 120hz but it is a lg gp850b ips monitor from LG and it allows me to run 1440p at 120hz but I just wanted to know which would be better for FPS games like Siege etc like if 1440p would have more of an advantage than 1080p as there is a lack of information around regarding that and all I find is things about pc about lower res is better for frames but the consoles are optimised to the best of my knowledge for any resolution up to 4k at 120hz.

So what's that meant to mean ?? All I wanna know is if 1440p is better then 1080p and I know my monitor is good etc I did heaps if research on it before

Multiples of 30 (yes NTSC is 29.97fps) are the only valid refresh rates under VESA standards, which Xbox is based on. These are the same standards all consumer electronics use.

The overclocking can throw those timings off and cause incompatibilities. We see it here every day.