I currently have PG278Q (the TN one) and have seen the PG279Q personally at one of the IT conventions in Taipei near the end of last year, as well as XB271HU (the refresh version of XB270HU), they have the PG279Q on both static display and for racing. I was in the market for an IPS monitor to complement my PG278Q, but the largest constraint was that the IPS glow MUST be less noticeable than PG278Q's BLB.
My immediate conclusion to the monitors are:
1) The response times on the PG279Q wasn't noticeable (they were demonstrating a racing game). Actually the only time I really notice blurring was when I switched from PG278Q and BL3201PT to compare the IQ of the two, but the blurring largely disappears after several minutes. So, IMHO, the response time difference between the monitors are mostly on paper. Some people may notice it more.
2) IPS Glow: XB271HU completely blows PG279Q out of the water on this one. The Acer stand allowed me to see a fully black screen, and my immediate reaction was that XB271HU's glow is comparable to that of PG278Q, but PG279Q was woeful. I concentrated on a black patch of a webpage (the Asus stand won't let me change the picture), and it was placed right next to a white background, which should make IPS glow less noticeable (due to perceived contrast), but it was actually MORE noticeable in that configuration than Acer's completely black screen. So for all intents and purposes, I struck off PG279Q as a possible purchase. I was completely unwilling to sacrifice black levels for viewing angles.
3) Price: PG279Q and XB271HU costs the same here, but cost is somewhat irrelevant to me at the price range (sub $1000), I was more than willing to spend extra on a better monitor.
In the end, I settled on BL3201PT. XB271HU might have swayed me if I was looking into replacing my PG278Q, but I wasn't, though if I was doing it all over again, Acer would be my first pick, followed by PG278Q, THEN PG279Q. YMMV as I have a greater tolerance towards TN and far less tolerance towards IPS glow.