For overwatch, should I get a pg348q ultrawide 100hz or a pg279q 16:9 165hz

If you are buying a monitor specifically for Overwatch 21:9 is cropped and actually reduces FOV versus 16:9 so the pg279q would be better.
http://www.pcgamer.com/overwatchs-new-219-support-actually-reduces-field-of-view/
 
I would think you would be better off with the pg348q in terms of gaming because of the very wide viewing angles you will have, I would certainly think that would give you more of an advantage than an extra 65hz will. I actually have the PG279Q and I love it, other than the horrible backlight bleed the majority of these new 165hz ips panels have. I would have gotten the pg348q if it wasn't 1300 dollars, which is in my opinion absolutely ridiculous. The PG348Q would aslo be harder to push than the pg279q.
 


Oh didn't know about that, something to consider as well. Overwatch did that to keep people with 21:9 monitors from having an advantage. But there are also many older games that wont support 21:9 either.