It's not hard to run 50 users on a modern high-speed 1U these days, and with Windows the licenses may end up costing you a large percentage of your total purchase price.
If OS X can provide directory and authentication services that are worth that, and can get the application developers to support that platform, they'll be worth considering.
Directory services are based on OpenLDAP with Kerberos (and not a Microsoft screwed-up version either). Actually, most of the services are standard UNIX or based on them. As far as development on the platform, there's Java, Ruby on Rails, PHP, Perl, Python, etc. without having to port anything, in addition to programming directly for the Mac in C, C++, etc.