We build for a customer a social network site, where we will have on beginning 1 Master and 2 Slave MySQL Server.
I'm not sure if we need to go with the Enterprise or use the GA Version. The only advantage for me would be the MySQL Enterprise Monitor, who we only get with the USD 5000/Server Enterprise version. Would you suggest to buy it, or to use a 3rd party monitor tool? Does anyone know any similar tool as the MySQL Enterprise Monitor?
You could also take a look at Kontrollbase. It is feature-rich and free and there is also an online demo. On the site a way down there is also a feature comparison with MonYog / SysYog and MySQL Enterprise Monitor.
Actually I would caution again MySQL enterprise without thoroughly looking into your options, unless you're using them for a service agreement or to meet an SLA. To many people think paying for an option automatically makes it more reliable.
I prefer community as it track record of bugs/fixes versus Enterprise is much better.
Personally I prefer nagios and cacti over MySQL enterprise monitor as it is easier to customize and far less costly.
I haven't used it but you can explore http://www.webyog.com/en/
The one major benefit of Enterprise is they will always backport fixes to a specific version of MySQL if you run into a BUG in production. With community version, they will not even look at the BUG unless you can reproduce it on the latest version that is out there.
Besides that, you can also get SLA agreements with Enterprise, but have never done that with them before.
IF you have a good in house DBA, Enterprise is probably not a necessity. Cacti/Munin can do a great job to graphing all MySQL performance and help you figure out how the server is being used and performing.