If you can afford the Mac Pro then I'd go for it. The Ray-traced rendering issues with non-compatible (not NVIDIA) GPU's isn't an issue any more because CUDA acceleration is no longer being developed. The Mac Pro is compatible with Mercury playback in Premiere Pro so that's where you should go. Choosing a system based on a deprecated feature is not a good idea. To tell you the truth I have never used Ray-traced rendering for a client. I've played around with it a few times, but for the most part, if I need 3D I will use C4D Lite that comes with AE and end up with a better product that renders much faster.
.....uh.....wait a minute. Although the single "ray tracing" effect has now been replaced with C4D Lite, MANY other effects and operations ARE DEPENDENT on NVIDIA ONLY GPU acceleration in PPro !! This "acceleration" allows for a TEN TIMES increase in rendering previews,or, in exporting timelines which contain : scaling, de-interlacing, AND a multitude of GPU accelerated effects that are provided in PPro !! I am NOT sure that " CUDA acceleration" is no longer being developed....I AM sure that AMD equipped machines suffer greatly in performance vs. NVidia GeForce equipped machines...you can see the test results on the video bench marking website : PPBM8.com !!
Many plug-ins may not benefit from the GPU acceleration,or, they may not "thread well", which means a system with more cores is of no benefit with them, ( i.e. the Xeon system )....you would have to investigate.
PCI based SSD storage is very tempting, as it is so fast....however a machine must be BALANCED in all its hardware for good,or, maximum performance.
As the top performing machines tend to be Windows machines that have many configuration options and a LOWER COST....I would carefully research before spending big bucks on an Apple product to make sure it had ALL that may be needed....especially with the rapidly approaching future which will relegate 1080p to a museum !!
At THIS time for best performance the general requirements are :
1. a FAST i7 CPU...the more cores, the better,BUT, the HIGHER the clock speed, the BETTER.....recommended : Haswell E 5960X on an X99 Mobo overclocked to over 4Ghz. This CPU has 8 CORES !...( 16 "threads" ).
2. PLENTY of FAST system memory, ( as PPro NOW uses MUCH more memory than in the past...32 GB is MINIMUM for 4K video and 64 is a solid start concerning AE, which will take all you throw at it, Recommended : 64 GB of the NEW QUAD CHANNEL memory on the X99 board with the Haswell E CPU
3. FAST STORAGE : as the X99 board has up to TEN SATA III ports AND a direct PCI connection to 4 lanes of high speed goodness, you have great flexibility in your choice of storage configuration to attain HIGH SPEED without having to resort to an expensive add-in RAID card !! I do not know if you can do this on an Apple machine.
Example : The Samsung SM951 as your media drive connected to an M.2 port that goes DIRECT onto the 4 lanes of PCI generation 3 speed...running at over 2GB a second read and 1.6 GB a second write SIMULTANEOUSLY...NO "one at a time" that SATA III does because of the "half-duplex" interface. Also, virtually NO latency on the PCI SSD...especially on the
NEWEST NVMe version that was reviewed TODAY by Storage review. This new storage technology is now available and it is revolutionary compared to the nature of mechanical spinning hard drives and SATA interface crippled SSDs. I would want to use it ! THEN, you can use all the SATA ports to create a high speed RAID 0 of SSDs for archiving and backup. Your
boot drive could be the SM951 with a RAID 0 of SSDs for media and the rest.....then, STILL you have plenty of ports for a complete backup solution.
4. SOLID NVIDIA GEFORCE GPU !!! The test results posted here on this forum by Eric Bowen under " Haswell E benchmarks" and posted at the PPBM8 website confirm the great performance of the top tier NVidia GeForce cards....at least one 970 is recommended.......the 980 is better, and the new Titan is best......having TWO of these GPUs DOES improve SOME, but not ALL, operations done
If it were ME....knowing that the software being used is at the CENTER.....I would want the machine which runs that software to be the BEST for the most reasonable cost. Make sure your Apple product can provide that first, before committing to spend...if it does, fine....but, will it handle the more difficult and high resolution video codecs coming very soon, if not here already ?