I find it difficult to provide much advise as you did not really share to many relevant details.
I use Fusion 360 on both OS. I have a high spec 2017 Macbook Pro and on that machine use Fusion 360 predominantly on macOS. However, I also run A Win10 installation in Bootcamp to use other CAD software and have done extensive performance comparisons. There is no appreciable difference in performance between Windows and macOS on the same hardware.
I personally prefer macOS over Windows.
My computer at work is a Dell Precision 7550 with 32GB of memory and a Quadro P2000 graphics card. Win10 obviously 😉
Nothing special and Fusion 360 runs fine.
Fusion 360 uses the CPU only for rendering/raytracing, so a better graphics card is not going to make too much of a difference. If you use Apple hardware then I'd definitely get a computer with a dedicated graphics card and not the on-board Intel only graphics.
It really comes down mostly what other things you are hoping to do with the computer.
One thing that occasionally comes up are screen scaling problems on high DPI screens when using Windows. based machines. I have not experienced this either in Windows or in macOS. Fusion 360 appears to scale perfectly fine on both.
I run a dual monitor setup (at home). Native retina display on the Macbook and a LG CX 48" 4k OLED Monitor.