The CPU is the center point. That's where everything happens, so you want the best you can afford. Quad core preferably, the more GHz the better. (Don't go under 2.3 today.)
If you use a lot of big sample libraries like Omnisphere or Kontakt libraries, you want also a lot of RAM. 16gb is cool enough.
SSD might be a good idea as everything loads much quicker with it, but they are expansive, so I'd worry about it later. CPU is more important.
At least 1TB HDD for data and you're good to go.

As for the MACs... You'll get nicer and better built case, but for the same money, you'll get less powerfull components. ...and even if the money isn't issue, Apple doesn't care too much about backwards compatibility. With El Captain version of their OS, a lot of companies had to adjust their plugins so they work after update of OS. Including Steinberg, who made whole damn VST standard. ...and that took time.