I'm a computer technician and have a bench PC for doing diagnosis, HDD repair, file transfer etc.
Due to being part of Microsoft Dreamspark I get a free license for Windows Server 2012 R2 Standard/Datacenter Edition.
Otherwise I can buy Windows 8.1 Pro.
What would you recommend?
Depends on the software you are using. I like using Ubuntu because of the flexibility but you lose access to a lot of mainstream applications as the performance under Wine is sometimes less than ideal.
If you go with windows 8.1 you may run into compatibility issues with other testing software (I have in the past)
If you use Windows Server, you may run into issues as it is for a server install.
For my work I use windows 7 and have XPMode installed. It means I get to use windows 7 and if the application does not work there I can open the XPMode application and it runs as if I am on an XP machine. I also can dual boot into Ubuntu if needed.