I faced the same choices you are considering. This was my experience.
I ended up getting the most powerful system I could - the HP Z820 fully loaded with the fastest processors.
PCs seem to have a much greater flexibility for GPUs in kind and variety, and numerous other modular external components, among other things. Not to mention the huge difference in price for the setup you get.
That part is very true.
The consensus among my peers is that Mac is more stable
That part isn't. (Meaning both systems have issues.)
Cinema 4D is all still CPU rendering unless you get a 3rd Party plug. Max will work with either Geforce cards or Quadro since Max has gone Direct X with the latest version. Premiere will function better with a Geforce 670GTX 4GB or a 680GTX 4GB than a Quadro unless you require 10bit Color Preview.