Thunderbolt, Magsafe, webcam, and built in (crappy) speakers on ATD can easily be replaced by multiple video input, USB 3 hub, rotatable arm, and audio out capability on Dell. Those are equally great features Apple display could not provide.
Glossy glass surface on Apple display is annoying for many people, matte display is a holy gift for the others. Do you want a display to do real work, or a shiny object?
Can your MacPro use ATD? Each has

logo on it, no? While that plastic fugly Dell display welcomes your MacPro just fine. Who's more functional then?
Yeah, the Dell works with any Thunderbolt Mac too because it has DP input, while shiny ATD is a $999 paperweight unless you have Thunderbolt computer laying around. I wouldn't waste a grand for something that spoiled.
In the end, you may consider each features even out for both monitors, and even after all that, the Dell still comes up cheaper.
On top of that, like I said, the Dell comes with 3 years warranty, no need of getting ripped by another AppleCare.
Rip off is a rip off no matter how you put a lipstick on it and play the angle