I have owned both of these monitors and also the asus vg278h which came out prior to the HE edition, aswell as the xl2411t and lg 23" 120hz, samsung + viewsonic 22" 120hz!, samsung b950 27" 120hz from a few years ago and countless others......yea sad i know
I'd say there is very little difference between the BenQ XL2720Z and the Asus VG278HE.
The Asus has more obviously "overly" bright colours, which i generally prefered over the far more "washed out" appearence of say, the xl2411t, but the xl2720z's are far more "balanced" overall in my opinion.
The benq xl2720z is the first 27" monitor that I've ever used where I can actually play competitively at a high level on Quake without feeling at a noticable disadvantage in regards to input lag, as from a responsiveness point of view, it feels pretty much like a benq 24" xl2411t/z. However, for literally ANYTHING else, both of these monitors are perfect and very responsive overall.
TL : DR : Both are great but if I was buying now...I'd go for the Benq xl2720z to be honest