I prefer running FileMaker Server on Mac Pros, but they are getting old in that the last one that came out was introduced in 2013. I had hopes maybe a new one would be announced at WWDC this year. But alas, it looks like it will not happen in 2017 according to this article. FileMaker Server still runs great on them, but if you want the very latest server hardware, you will need to look at a Windows server at least for this year. Hopefully a spectacular new one will come out in 2018!