I've never heard of "Quad Sealed."  Kinda sounds like another Burris Gimmick, kinda like their "Posi-Lock" gimmick.  Every Nikon, Leupold, Elite 4200, and other similar scopes I own don't have "Posi-Lock" and they have excellent tracking and keep their zero perfectly.  
As for the O vs. Quad Sealed, take a look at the other scopes you mentioned - Leupold, Elite 4200, Zeiss Conquest, etc.  These scopes are top notch scopes and nobody has any complaints about them. 
As for clarity, resolution, etc. is concerned,  I don't know about the new Leupold VXIII, but it's hard to beat the optics found in the Elite 4200 and Zeiss Conquest.
I own a couple of Burris Black Diamonds, and I'm not impressed with them.  For the money, I'd get an Elite 4200 over a Burris BD every time.
Zachary