If you must stay on a Mac, like me, look at building a thread ripper render slave if you are using The C4d standard or physical renderer.
AMD has amazing bang for buck. You can get a decent deal on a thread ripper machine building it yourself, or ordering one with a bare bones GPU from a shop.
If you are using a CPU bound 3rd party solution like Corona, also get a thread ripper as a slave and stay on your Mac.
If you re going the GPU route and must stay on a Mac, the regular iMac with 2x 1080TI in eGPU enclosures, but do not upgrade to Mac OS Mojave. CUDA does not work there yet and nVidia is supposedly working on it.
If you are going to use something like Cycles 4d, that also works with team render. You can put 2x GPU on your Mac, and throw some GPUs in a PC render slave and do the same thing. At that point, get the cheapest thread ripper board for the PCI lanes and throw 3-4 GPUs in it and keep it as a slave.
The nice thing about the slave is having a backup C4d machine if anything goes wrong.
As for Mac/PC, I know this hurts… I’m a Mac guy. But there are so many reasons I regret buying my Mac. uRender is PC only. The full Quixel suite is PC only (Mac version announced in March, but no news since then). Apple dropping support of Open GL, Open CL, etc. The new nVidia 20XX series only works on PC for now. nVidia cards don’t work with C4d Pro Render on Mac, making them useless for anything but 3rd party renderers. Apple eGPU solutions don’t work in bootcamp. Apple doesn’t have AMD as an option for cheap multithreaded machines. The list is growing.
I do not like windows after my experience building my own PC and having tons of motherboard/bios problems. But I freely admit that PC offers so many benefits that I am considering selling my iMac.
EDIT - why does my name say “pro” best to it? I’m no pro. I’m a hobbyist at best!