Which webserver is better for production for RoR hosting?
for example: mongrel vs apache2?
can mongrel does something that apache2 can not ?
mongrel is an application server, while Apache 2 is a web server. They aren't competing with each other, but working together.
Having said that, you should forget about mongrel and go take a look at Phusion Passenger, which is an application server that will work with either Apache or Nginx (another web server).