Emulator machine, what OS?

I have purchased a Zotac ID91 with 8GB of ram and a Samsung EVO 840 SSD.

Going to use it as a dedicated emulator machine for my living room.

Will be installing the following emulators: ES (Nestopia), SNES (snes9x), PSX (ePSXe), Sega Megadrive, Master System (Kega Fusion).

As frontend probably going to go with GameEx (v14), that's why I signed up here.

GameEx looks awesome and probably does everything I want. Need to register a full version though.

At the moment I don't know which OS to pick. Windows 7 or 8.1 (x64).

Which one would be best for a dedicated emulator machine and why?

Thanks in advance!