If your only use for the system is Flame then the Linux systems seriously outperform the same spec Mac Pro. My old Z820 with a GTX1080Ti was outperforming the new Mac Pro on the benchmark test, as well as some other real world tests I have done. However, if you are not Linux-savvy and are not prepared to get you hands dirty in the command line then it may not be the best choice.
If you plan to run other software on it and want something both easier and more versatile, then get a Mac Pro.
Oh, and for those who say the Mac Pro is more expensive, if you compare it to a similar spec PC from an OEM, it comes up at a similar price really. If you’re happy to build a Frankenstein PC though, it will definitely come in a lot cheaper but you are not comparing Apples with Apples, pardon the pun. And you will currently struggle to get a decent GPU for the PC too.