The one piece of wisdom I can share is that video RAM is becoming more important for some editing and compositing programs so make sure the video card has enough. The minimum used to be 2GB but lately i've seen suggestions that you need at least 4GB of video ram.
Start by checking the recommended specs from whichever software you plan to use—they will get you to the right ballpark at least. This is only my experience but I followed Blackmagic's recommendations when I bought a computer to run Resolve and it really narrowed down the number of decisions I had to make.
As for Mac vs. PC, I've always just used whatever my clients use. They're the ones who pay me. Hopefully others will have more logical reasons for picking one over the other, but that's my reasoning.