Update, here's my thoughts for anyone who may be interested.
I refused delivery of the Dell S2716DG because the Acer @ £383 was just unbeatable - so no comparison with the 2 monitors I'm afraid.
Both my monitors have been calibrated with DisplayCal + Spyder5.
I've got the XB271HU sitting side by side with my X34a and I've got to be honest I almost returned the XB271HU because when you see the colour and viewing angle compared to the big IPS panel then you see why IPS beats TN for image quality hands down. However, having said that the 144Hz Gsync gaming with the TN panel is really smooth and the image quality and colour etc during gaming is really good - it's desktop/internet use that I find the TN panel really lags behind - the IPS panel just seems to have a more balanced colour reproduction and the TN panel is a bit sharp and bright on my eyes. The TN viewing angle is also really annoying - again, maybe I wouldn't notice it so much if it wasn't side by side with a X34a.
As far as gaming is concerned, I loaded up GTA V and took a screen shot at the same place at both resolutions 3440*1440 & 2560*1440. I then had the images side by side on their respective screens and the difference is not that big, certainly not a deal breaker. When I tested the desktop image quality I simply opened a web page on one monitor and then dragged across a duplicate to the other. This is where the XB271HU is really let down as the X34a is simply miles better on the desktop.
I have to be fair here - I'm comparing a £900 screen with a £380 screen so of course the X34a is better, but to be honest I couldn't go back to desktop/productivity use on a 27" TN panel. With this set-up, I have the best of both worlds and I have to re-iterate; for the money this little 27" panel is a cracking buy and for me in particular it's great to have a 16:9 screen for games that don't support 21:9 and with 144Hz Gsync into the bargain (I'm already finding games that I prefer on the 16:9 panel (e.g. Fallout 4)) - so yeah, I'm pleased with this purchase.
Here's some images, I'm not sure what value they bring to this "review" but just to give an idea of how I evaluated the screen.
