It's strictly a budgetary decision ...... therwise bigger is usually considered better. But ya gotta keep in mind your peripheral vision .... considering how far ya have to sit back to see a 81" wide field of view on the 30"? Can you sit back that for at your workspace ?
However be aware, that while the Dell IPS panels are "the thing" for accurate color production, and an absolute must for photo and video editing, be aware that many people don't like them for gaming. Game developers tend to design their color schemes to fit the largest market segment ad that is TN panels......when viewed side by side, many peeps actually prefer the oversaturated TN panels. Before you invest $4500 on monitors, you should be aware this.
The other thing as I mentioned in your other thread is .... are you interested in 3D Vision . If ya wanna go for the 3D thing, you'll need 120 Mhz monitors