You mean with the nMP Quad Core or iMac 5K? I'm so used to always getting a laptop that picking the right desktop solution is confusing me.
Classic Mac Pro (2009 or later) can be upgraded in ways that for some is a better fit and can be configured for some applications and work loads to be (subjectively?) more applicable.
Can do 5k, can take a full discreet GPU of your choosing rather than limited choice and a mobile version of GTX or AMD.
Where the iMac (and I have always had trouble even as they have improved) of calling these a desktop computer, it uses more of a closed limited access and motherboard adapted from the world of laptop and the cooling requires (correct me if I am wrong) more fan rpm to maintain cooling (and 27" has more room and better cooling).
Where the iMac would likely be maxed out, you are just getting started with any Mac Pro.
If you use Final Cut Pro's latest then you might want an 8-core (custom Xeon with higher clock than the BTO that Apple offers) and $1,000 extra for D700s (two year old 79xx series?) and even here a "cMP" with dual AMD R9's and dual 3.4GHz processors might kick some butt.
You can start off with a 2009 Classic Mac Pro for $500.
It can take one or more PCIe-SSD blade drives capable of over 1000MB/sec and 4x whereas your laptop would use a 2x. The 1TB SSD in the nMP (6,1) is another one of those $1,000 BTO options too, so again people like to do it themselves on their own easy enough - and can do upgrades as needed, and as their $$ funds permit rather than all at once up front.
48GB of RAM? 96? maybe 128GB for huge Photoshop graphic projects?
4x6TB of internal storage that avoids clutter, cables and power adapters or the costs of Thunderbolt2. So storage is another area the Classic Mac Pro shines. 2TB of SSD-PCIe and the 24TB of storage along with one state of the art or two graphic cards - that may require a 2nd 450W external or extra PSU (some can go in the 2nd lower optical drive bay, again a win for cMP design - still has a place for optical drives).
That is in part how I come to see making a subjective choice in which one fits the needs of the person and applications.
The only time the Classic is a problem, for me, is having to lift and move it! my back is older now than when I bought mine (still going strong too) almost nine (9) years ago (Mac Pro 2006 1,1).