a 2013-2019 Mac Pro as follows: Mac Pro 2.7GHZ 12-CORE 1TB SSD 128GB RAM, 2 x D700 6Gb
I think that the iMac would probably be better on a day to day basis, but the Mac Pro would be better for long stints of TwinMotion rendering? I've seen some benchmarking that suggests that you do have to load up the old Mac Pro with maxed out RAM to get the most out of its 12-core Xeon. I also think the Mac Pro will deal with the increased heat load from Twinmotion giving it a workout.
I'm currently running a 2013 era iMac with a 3.4 GHz 4 core i7 with 32Mb of RAM, and a 2Gb video card - so I'm pretty sure either machine will be a gigantic upgrade for me. As soon as I start Twinmotion on this machine, its fan goes to "11" trying to cool itself!
As you are apparently a long-term Mac owner, you know that Apple will drop support for new operating systems on older hardware sooner or later. A loaded 2020 iMac would make more sense for a longer usable life in that regard.
As far as TwinMotion - note that you can add an external GPU (eGPU) to the iMac via a Thunderbolt port in the future if you find you need better graphics performance, including multiple GPUs. The extra heat of course will be handled by the external box at that point.
This benchmark, for example, shows very similar graphics performance with the same eGPU in both MacBook Pro 13" and the much more powerful MacBook Pro 16". As noted there, the 13" falls down on pure CPU (ARCHICAD) performance). These tests were done with a single GPU in the eGPU. https://barefeats.com/macbook-pro-13in-vs-16in-5600m-and-egpu.html
That same site has another set of benchmarks - again of a 16" MacBook Pro which I know you're not looking at ... but the Blender benchmark includes times for an eGPU with one GPU card and another with TWO cards. The test with two cards shows half the time as one card... hence another option for TwinMotion if the iMac alone does not perform well enough for you. https://barefeats.com/16-inch-macbook-pro-egpu-added.html
One of the forum moderators • AC 25 USA and earlier • MacOS 11.6.3, iMac Pro