If I were in your positions, I might just save some money and wait for the Asus ROG 4k Swift. It's a UHD Gsync
with an IPS panel. There's no technical tradeoff there. The only downside would be the higher price, but the new 4K Swift won't come out for a couple months, so that's where the saving comes in.
But if I needed something now, then I'd calculate how much time I spend doing various things. Like if I had another computer with an IPS monitor for photo editing (assuming you do that), then I'd definitely get the Acer - no question. But if I just did a few hours of gaming here and there, but mostly used my computer for other things, then the Dell would win out.
Have you played with demanding games and compared the difference between Vsync on and Vsync off?
On a 60 Hz monitor, dropping below 60 fps with Vsync on makes the game s-s-stutter; above 60 is smooth. But if you turn Vsync off, then you get really ugly tears. Gsync eliminates that tradeoff entirely, so you have smooth, tear-free games even below 60 fps. Great technology. It's even better, however, on an IPS panel
Here's a link to it as displayed at the Consumer Electronics Show in Las Vegas this past January: http://rog.asus.com/393642015/gaming-monitors/ces-2015-rog-swift-pg27aq-27-inch-4k-lcd-with-gsync/
Linus from Linus Tech Tips also did a quick video on it while he was there: https://www.youtube.com/watch?v=1QiQ6y1jJ5A