I am now with the old Mac Pro (the silver tower, NOT the new black one), with a W3690, 32G RAM, and pair of HD7950...
This config allows me to use FCP X to edit 4K video without any problem. And play games in Windows 10 with reasonable performance (crossfire enabled).
From the benchmark, it shows that the new Mac Pro can do the same thing, and able to perform better than my 6 years old machine. However, it seems quite a few members here have trouble to deal with the GPU drivers in Windows.
In my config, everything is well recognised in both Windows and OSX. Apart from I must disable the GPU's EFI in order to make the machine can boot with crossfire enabled in Windows, there is no other software issue at all.
In fact, for gaming, there are more GPU choices in the old Mac Pro (all the way up to Titan X). However, for anything above GTX680, it will not be self flashable, and may not be supported natively in OSX (require Nvidia web driver).
So, it's your choice to buy the most up to date stuff, get better performance, but deal with the drivers issue. Or get the older stuff (which is cheaper as well), with better software support, but relatively poor performance (still good enough to game at 1080P).
In my experience, it's possible to have one machine to do both job decently. The problem here is in general AMD card works better in FCP, but Nvidia card works better for gaming. Also, FCP can take benefit from more cores CPU, but gaming require better clock speed rather than more cores. Since you can't have both at the same time, you have to decide which performance is more important for you and get the correct CPU / GPU accordingly.
TBO, if money is not an issue. Why not have 2 machines? Which gives you best performance in both FCP and gaming, also able to do 2 things at the same time (e.g. play games when FCP is rendering).