It used to be that you could decide between two computers by how much room they took up on the desk (or floor) or how loud or quiet they were under a heavy load. Those issues don't really come into play here. Both the Mac Pro and the iMac are stylish and take up minimal room on the desktop. And from my time with both of them, they're both equally quiet under operation.
That means the decision really comes down to absolute performance vs. absolutely-best screen.
There are those that want or need the best performing computers, and the desktop form factor — with its always-on power and components designed without mobile constraints — is their best choice if money is no object and performance is key. The iMac's screen advantage won't last; eventually, third parties will offer competitive displays that will meet or beat what the iMac offers today. But you'll have to wait for that to happen.
If you're trying to decide between a Mac Pro or the iMac, ask whether your software supports the Pro, and whether the speed gains you'll see with the higher-priced hardware is worth the extra cost. If you don't need that absolute level of performance, the over-all speediness, comparatively lower price and the absolutely gorgeous display, makes the iMac 5K a tough value to ignore.
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