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