Well, it's complicated, but the main reason is that since our premises are just around the corner from one of the biggest Apple Stores in the world, we can replace any of our servers literally within minutes if they fail. This makes me sleep a lot easier at night.
Also the savings on the electricity bill are not negligeable either.
>> our premises are just around the corner from one of the biggest Apple Stores in the world, we can replace any of our servers literally within minutes if they fail
I sure hope you don't have a server failure outside of business hours :-)
I think we're prepared for that too. First, we have a few currently unused previous generation Mac Minis lying around that could be used temporarily. Second, since everything we need (Apache and PostgreSQL, minus Mono) comes preinstalled with OS X Server, these machines can easily take over each other's tasks temporarily, e.g. the database server can become a web server too until we can get a replacement. It's not ideal but it'd do during off-peak hours. Fortunately, the Apple Store is open even on Sundays so even in the worst case scenario we could get a replacement within 12 hours max, which is not necessarily the case with, say for example, Dell servers.
You could still run windows server on those though..?
I could (and indeed that was what we were originally planning when we decided to go with Mac Mini Servers), but I simply don't need to. I open the packaging, put Mono on it and it's ready for work. Why would I waste time with Windows Server if it's not needed?