I have a older 22", 24" and 27" Ultrasharps at my desk and I've been messing with those Asus 24" IPS displays and here's what I can say: For day-to-day work, the quality of my older Ultrasharps and the Asus monitors is subjectively very similar. They're all worlds different from crummy TN panels, but I don't have anything sitting around to calibrate them to really know. I think the Dell screens might be somewhat brighter, but mine are older so they look very similar to me (I keep them at about 60% brightness and use the 6300k color temperature setting).
Honestly though I'd be more than half tempted to look at two 1920x1080 Asus screens for what that Ultrasharp costs. If vertical pixels matter, find a couple monitors that support vertical orientation (the Asus screens I have don't, at least not on the stand they come with) and use 'em that way.