I have both. I personally can tell and feel a difference between the two. Before any numbers hit, you can search by date and I had mentioned a few times around here I could tell the difference on the VG259QM between OD and ULMB. I am hyper sensitive to motion, I don't get motion sick at all but watching a movie or something I see the frames pretty obnoxiously. Any sort of latency I can tell 100% (this is likely due to a few workflow reasons). There is probably some medical person that can explain but I don't seem to catch ghosting or blur at the level these are getting at like some. My guess is its likely my "gaze" when gaming or me zoning into the things around or something else is going on with the frames.
For me the differences no specific order,
No contest out of the box in color accuracy.
Higher refresh rate matters to me, may not for others. People can argue what they experience but at the end of the day you know what you know and each user would have to decide and let their biology determine what is going on.
I don't know what is what number wise but on CM105 VG259QM feels incredibly slow when it loses frames. If I'm playing a game and take a significant drop in FPS 0 chance I won't know. Its tough with that display. The PG259QN isn't in the same ballpark in this regard *to me*. If the numbers say other wise I will agree and scratch my head. I won't on the 280/360 refresh rate debate if that is a thing.
As wild as it sounds I believe I'd grab my Omen X 25F over the VG259QM for comp if the PG259QN went down. It feels consistent throughout the refresh rate. I'm taking the PG259QN over all period.
The VG259QM is easily the monitor I would recommend between the two, its an incredible value and performance. If you want the best of the 2 for me its not close, its the PG259QN. This will and should vary by individual. All of our eyes, brains, reaction times are different. I really only play competitive shooters and at times have a lot financially on the line so there is that. Try both if you can before you buy. I know its easier said than done.
My only competitive game is CS:GO and it's CPU hungry one, so I'd rather invest in a new platform first, and then add something like RTX 3060, but these are still quite distant plans. I don't think 360 Hz IPS in current price is worth the difference from 280 Hz IPS anyway (and it's too little difference in your recommended upgrades as well), and another problem you point out is mouse polling rate: there is no way I switch back to wired mice, so I will always prefer 1000 Hz high-end wireless over 8000 Hz wired (hopefully wireless mice will also get higher poll rates), which makes it a better idea to stick with 240/280 Hz for now, probably.