The customisations they offer do not give these machines modern hardware per se. The CPUs are of the same generation that the original Apple Mac Pros of the corresponding times - they are not new Xeons. Far from. The only modern xeon in any officially supported Mac is in the iMac Pro.
For that reason, CPU wise, the iMac is better. And by that I don't mean that the 12 core won't outperform the i7 in heavily multithreaded tasks. I mean that the i7 will have each of its 4 cores, much, much faster than each individual Xeon core (more than just the clock speed difference too), and anything that doesn't push all Xeon cores on at least the 8 core Westmere will not at all be worth it, as the iMac will run faster. – To make it worse, all GPUs you could plug in these will be bandwidth limited, as all but the cylinder (which is not customisable), use PCIe 2.0. You can still get it with a faster GPU than the iMac, but it will not perform as well as it could, and not as well as you've paid for it to perform
The iMac is most definitely the best option until the next Mac Pro is released. You also get a lovely 5k display with it to edit on.