As an IT guy, I've seen ASUS grow as a company and appears to be shooting for Samsung level status. Benq is a smaller company. Benq monitors over the years have been value based choices to save money. That's my opinion and I'm sure Benq makes great monitors that cost more. But keep in mind that ASUS and Benq are not necessarily shooting for the stars in the photography / graphics design market. They are shooting for the mass market and the gamers. They are making monitors that scream fast refresh rates.
While the statement above says nothing about the actual quality of the particular models you are considering, if you are dead set on buying from those brand names, I would go with ASUS. ASUS has a pretty good reputation in the market for most of their products including LCDs.
Of course, for a high end graphics quality display like you are shopping for, I wouldn't consider either company. I would stick with the higher end products. HP, DELL, EIZO, NEC, etc. One way to get a high quality display without breaking the bank is to buy a refurbished (or not) DELL U2410
IPS display. It's a bit older but it's a very well respected display that can be obtained for around $200 - $300 now. When you get it, you'll understand what a true quality display is. It was about $1200 back when it came out in 2009. The newer version of it is the U2711
which is around $500 - $700.
Most of the web comparisons are biased but here is a decent list of better options.http://reviews.squidoo.com/photo-editing-monitors