The U2713HM is a better monitor overall, as you know both do have issues with quality control though.
The U2713HM often has bad backlight bleed and noticeable crosshatching while the PB278Q often has bad backlight bleed too, as well as dead pixels. Dell have much better customer service, if you have an issue you can be sure that you will be given a replacement ASAP, while Asus has a track record for refusing warranty claims.
The reason they are so cheap vs a lot of other named monitors is because if these quality control issues. While it does still have issues, the Viewsonic VP2770 is a better monitor than both (arguably even has better image quality than the Dell) and has less quality control problems, it is a bit on the pricier side though.
If it has to be between the Asus and Dell then a vote for the Dell because of the superior image quality and customer service thumb.gif
Edit: The Dell also has much better calibration out of the box, which is great if you don't want to shell out for a colorimeter.