Are their any utilities on OSX that can show you what the instantaneous usage of each core is?
Under Windows 7/8/8.1, I found that Cubase, while utilising all cores, did favour one more. If that one gets overloaded, it doesn’t matter how many other cores you have.
That is why I would favour more speed rather than more cores, but as a bias, rather than being fanatical about it. However, one can use local VEPro instances to ensure VST(i)s are assigned to their own cores or at least to the least used ones.
Certainly, under Windows, this means that using overclockable consumer CPUs, like the 6-core i7-4930X, are perhaps a better proposition than using a Xeon 8-core E5-2687W v2 or a 12-core E5-2697 v2 (which cannot overclock), especially since those latter CPUs are several times the cost. If the CPU usage looks like it is closing in on the limit, one can just apply some overclocking. However, the new MacPros, being all Xeon, do not offer any overclocking, so whatever you choose for a clock speed is all you’ll ever get.
From what I have seen in written and video reviews, the new MacPros would seem to be designed for video-rendering, where it really benefits rendering turnaround time by utilising all cores. It makes for predictable processing times, but the use of Xeons means that the only scaling direction is time, in that any extra processing just takes longer.
With digital audio, the ability to handle processing peaks in realtime is paramount. At least VSTis can be farmed out to slave computers using VEPro, but that still means that all plugins must be handled on-board. If you like the UAD plugins, you can at least use the Apollo via Thunderbolt, or the UAD2-Octo via a TB-PCIe cage, to alleviate the CPU load.
I would suggest that you really map out your expected usage of CPU into the future to be covered by your purchase, otherwise you may be caught out, with no cheap way out. If you use a lot of samplers, maybe going for a lower end MacPro, so your desk has the cool new look, and having a hidden Windows ‘grunt’ box for the VSTis may be the better way to go. At least with the latter, you can upgrade as required, or even add another slave.
Really, if you are following the Mac route, the MacPro is the only higher-performance option with a future open to you. Plan carefully.