One thing to consider: are you using any AfterEffects plugins that are using CUDA?
I've worked with people that used trashcan Mac pros with external GPUs in thunderbolt 2-PCI cases because it accelerated their workflows 20x due to CUDA acceleration.
If you're using those plugins, the Mac Pro 5,1 with the 980ti might be faster than the even the 28 core Mac Pro can do in Software.'
I used to have a 5,1 with 980ti, but I'm on iMac Pro now. I don't need CUDA but I sometimes see greyed out buttons and slow performance in applications when people try to do something on Macs that don't support CUDA. I my opinion, it's a huge pain that a lot of things only support acceleration via CUDA and NVIDIA built their monopoly, but if you rely on those Plugins there's no way around it.
With the 7,1 you won't be able to use CUDA even with an external GPU, because CUDA isn't supported in Catalina anymore and the probably 7,1 doesn't run anything before Catalina.
Another offender is Blender, which is supposed to be all Open Source, non-proprietary and multiplatform, but the Blender Foundation chose CUDA as their platform of choice for GPU acceleration. This means that when using certain Blender features a Mac Pro 5,1 will be faster than a $20k Mac Pro 7,1.