PHP and mysql are dead stupid easy to get started with. An mvc framework like symfony/kohana/zend/etc. can help speed dev time for your PoC, but most of them are performance hogs so you'll probably have to buttress that (APC, memcache, etc.) if you do end up having to scale. All this stuff's widely supported and retarded easy to deploy usually.
RoR is a bit more of a moving target at the moment. Which means docs can/do lag the bleeding edge quite a bit, standard OS distribution packages are frequently out of date (or just out right retarded e.g. debian's disabling of rubygem self update), and the best practices for deploying and scaling are in a state of flux. (I think currently the recommended way is phusion's passenger/mod_rails or whatever it is, for apache/nginx anyway. Or has the state of the art moved on again?)