impossible to say without knowing your hosting needs.
if you are hosting a single site with OS X Client and its doing what you need, then switching to 'OS X Server' isn't going to do much for you.
The unlimited version of OS X Server us unlimited 'file server' connections
Web connections are unlimited in OS X Client, OS X Server and the unlimited user version. There are no limits (other than the computers resources) on apache built into any of those.
in general though, you'll have more features/options and a more recent version of apache/php/sql if you go with a mamp package.