Q We need to replace our Xserve, one of the last to be sold by Apple. Given the uncertain state of OS X Server should we perhaps switch to a Linux or Windows server?
A The last Intel-based Xserves shipped with OS X Server version 10.5.6 or later, which is very different from Server.app 5.0.15, the current version for El Capitan. However, your Xserve is capable of running El Capitan and that version of Server.app.
Apple also still provides support for this model, although supplies of spares are not always plentiful. If your server has had a hard working life, now is probably the best time to replace it, before it suffers significant downtime.
Take a careful look at Server.app, and see whether it provides the services which your users need. In some areas – such as support for developers working with Xcode, and for iOS devices – Server.app is far more capable than the older, much more expensive versions intended for Xserves. Supporting common services, such as file sharing, and group tools such as calendars, it is now much easier to use and to administer. If you are already working in a mixed platform environment, then other server operating systems may make more sense, but for mainly Mac networks only OS X Server offers complete support for OS X and iOS clients.
Your choice of hardware is also much more open. If the amazing six Thunderbolt ports on a (new) Mac Pro are best suited to your storage, then that looks a very good option. However, even the humble and very cheap Mac mini is capable of supporting a hundred or more clients with core sharing, mail, and web services. For the cost of a Mac Pro, you could spread those services across several minis, in a compact and efficient array. There are also rack mount solutions available for the mini, such as that from Sonnet, if you still want to keep existing racking.
Updated from the original, which was first published in MacUser volume 27 issue 11, 2011.