In all reality, let's say someone were to build "simple" web applications.....by which I mean...a twitter clone etc., would it really make that big of a difference if I chose Clojure or PHP vs. ROR.
Maybe I am ignorant about the nuances of these platforms, but from a birds eye view, they all look able to perform the same tasks.
If you're not doing the work yourself create wireframe and/or spec of the project and talk to a few programmers. If you are doing the work yourself and getting started maybe how easy the language is to learn might be a factor.
Also if you are building a Twitter clone I'm assuming that you'd also need a database -- so there might be more things to think about as well.