Just to weigh in I've bought pretty much every generation of Apple's Mac Pro machines.
In 2015 I picked up a maxed out 15-inch Macbook Pro and since I benchmark every machine I get I was able to determine exactly what I gained/lost with a Mac Pro vs a Macbook Pro.
Normalizing my tests to the Macbook Pro as a base line of 1.
The Mac Pro 2013 (6 core, D700, 64GB RAM, 1TB SSD) in single threaded tests was 0.87 times the speed of a 2015 Macbook Pro. In multi-threaded performance it was 1.18.
I had a 12 core briefly and that was 0.77 times the speed of a MBP in single threaded tests. It was however 1.66 faster in multithreaded speeds.
In gaming those dual D700 graphics chips allowed the Mac Pro to surpass the Macbook Pro by nearly 5 fold. That was when using BOTH graphics processors in Crossfire mode in Windows. In OS X only one GPU is used for 3D apps which means you get a x2.5 fold improvement over a 2015 Macbook Pro.
Where the Mac Pro shines is in having a lot of Thunderbolt ports, in remaining very, very quiet under long periods of load and finally in looking drop, dead gorgeous...
It is also superlative in redistributing wonga from your coffers to Apple's...
Can't recommend it today, it's a very, very poor investment.
I briefly tested a maxed out 27 inch 5k iMac and to be honest that isn't too far off a Mac Pro. It's about 10% faster than a maxed out MBP in single and multi threaded apps and about 1.72 times as fast in gaming.
TLDR; If you need OS X the Macbook Pro or iMac are a far better value purchase than a Mac Pro and surpass it in every performance metric except gaming and very multi threaded applications.