You won't find many companies that offer dedicated OS X Server machines (in this case: Xserves), so this could already end the discussion before it's even begun.
Dedicated Linux machines are usually cheaper to get and they do out-of-the-box what you need/want. And they usually come with a nice web-interface (e.g. Plesk).
You can also give Windows Server a thought. It can also be administered remotely via Plesk, if the provider offers that, or through a Remote Desktop Connection -- which works great.
In my experience, Windows Server is a very reliable and robust solution, but more comfortable to use and administer than Linux. Also, you might want to go beyond just having eMail, but also want to use a real groupware solution like Exchange + Sharepoint.