I just went through this process... Been a heavy Apple user for 10+ years... got tired of the terrible technology choices Apple is making in regards to the product line. The lack of a new pro machine with NVIDIA GPUs is really the one that sent me over the edge. We can't keep using 8 year old Mac Pro tower workstations, IT dept prefers to update our equipment every 3-4 years. Most importantly for us, If you want faster rendering for most scenes, you'll want to use one of the GPU rendering options, and for an integration with FormZ, that means VRAY, or Maxwell, which both require NVIDIA GPUs for CUDA rendering.
I love my new workstation. AMD 1950x 16cores 32threads @ 4.0GHZ! , 32gb RAM, 1TB NVME SSD, GTX1080 TI + (2) GTX 1070s for incredible OpenGL Shaded performance + Fast VRAY rendering!! I already have (3) perfectly awesome 1200p monitors, so the price was much MUCH less than any Apple product, and the speeds for rendering are not comparable at all, even the stupid iMac pro would be much much slower. I personally do not like the glossy screens, nor 4k, 5k, whateverk. Multiple monitors is much more efficient workflow for me.
I really love OSX, and well, Unix/Linux in general. But the lack of true Pro hardware choices from Apple was really starting to hold us back as 3D design professionals. Honestly, I kind of like Windows 10, it's way more OSX like and very very customizable. I'm fully accustomed back to Windows now and I don't feel like I miss OSX at all.
Whatever you choose, FormZ will work great on new hardware. If you are a heavy user with complex modeling and rendering, you will appreciate Windows much much more for the faster hardware afforded over there. If you are a lighter user, you will find Mac still works fine, but it is slower hardware, and it runs slower per dollar than that of a Windows machine. It's up to you to decide how important that is to you.