Hi,
When you say powerful, what do you mean? 15 simultaneous layers of 4K playing across a composition of 15,360 X 2160, outputting to 3 sources?
I would think for most people a 6GB nVidia GTX1060 GPU, a solid state hard drive, 16GB of Ram, and a 6th-8th gen i7 would be a great machine to work off of.
Some might say that's a "powerful" machine.
In my opinion, you will get much more hardware and performance out of a PC than you ever will with a Mac (Much different than it use to be years ago).
This is a very subjective question as your needs may not be the same as mine. So it would be better to describe your day to day use, and also list your "biggest" show use, that may happen one or two times a year. This will help determine your needs vs wants. It will also help find a PC in your budget.
I've been running some very large shows (IMO) off a 6GB GTX1060 for about 2 years now. I didn't feel the GTX1080 offered that much more performance boost vs the GTX1060 for the price difference (about $2k), so for me, it was a needs vs want battle to find something that fit my budget well (especially when buying large quantities of machines). With that being said the new 8GB GTX1070 with Max-Q i just tested about doubled the performance of my GTX1060 and the GTX1080 I demo'd 2 years ago, and the machine with the 1070 is the same price as I paid for my current setup.
Just let us know what you do and your budget and I'll try my best to point you in the direction I would look at