Topline: I think in your situation you'd be better off using plain windows (not windows server). A few points:
1) MC is not supported on windows server, and it's not recommended. Some folks have had good luck, but others have had myriad, mysterious problems due to parts of windows that MC relies on being missing or different in windows server. So if you go windows, I'd recommend using a consumer windows build (I ran my own server that way for a few years with no issues at all).
2) Due to MC's difficulty with cross-platform path names, windows clients of linux servers and linux clients of windows servers will not have a perfect experience. It may or may not matter depending on your file types and general needs, but, for example, .ifo/directory rips of DVD's will not work correctly with cross platform server/client scenarios. .MKV rips work fine, as do most other file types. Sometimes the lack of direct path access can trigger transcoding when no transcoding is necessary, but that issue can be addressed by careful attention to settings. I currently run my server on Linux with a mixture of windows and linux clients and it mostly works fine, but I had to choose a side because my clients are mixed; if I only had windows clients I would not have migrated my server to Linux. I haven't heard of any fixes on the horizon for the pathing issue.
3) MC for Linux is not as "feature complete" as the windows version in ways that affect it's use as a server. For example, the live TV functionality isn't working on Linux yet, and the Linux version of MC (mostly) can't serve live TV tuners to clients. That's on the agenda for MC22, but is not there yet. Theater view for Linux just made its debut as a testing branch, which is good news as prior to that there was no way to edit client theater views with a Linux server. That's not in the mainline stable builds yet, but should be very soon. There are a handful of other areas of limited function, but TV and Theater view (until it arrives in mainline) were the main holes. That's another point for windows for now, but feature parity is coming.
4) A linux server is (in my own experience) more stable and more tolerant of underpowered hardware than a windows server. My windows server worked well, but required frequent maintenance and weekly reboots. I reboot my linux server two or three times a year, typically when I need to add another hard drive or clean the dust out. So that's a point for Linux.
I love linux and prefer to use it whenever possible, but I don't think it's likely to be the best choice right now for a setup that only has windows clients. Next year might be a different story. Hope this helps.