Ruby is a programing language. Ruby on Rails is a web framework. That is not a pointy-headed distinction, because the reason Rails is vastly superior to regular ol' PHP for certain types of prototyping tasks is because of all the stuff it has built on top of Ruby. For example, ActiveRecord, which will save you from having hard-coded SQL calls in the middle of your prototype.

There is a certain amount of conceptual overhead which happens with Rails projects that doesn't happen with PHP. Deploying Rails apps is still an order of magnitude or more harder than doing PHP apps -- even with Passenger, which supposedly fixed deployment. There will almost certainly be more configuration in a Rails app.

On the plus side, the Rails and Ruby communities have wide and deep amounts of OSS code for you to use to accomplish typical web dev tasks. The PHP community is, ahem, well, I'm sure they're wonderful people and if you're working on Wordpress they have huge amounts of well-packaged code available for you to use to accomplish your tasks.

You don't mention if you're a programmer already. PHP is well-suited to folks with minimal programming ability. I would think doing Rails without having strong object oriented programming skills would be a recipe for disaster.