The biggest problem I've faced so far writing web apps in Ruby is 
finding a webhost that supports ruby! PHP was designed (more or less) 
with web in mind, and has been around for quite a while. It is very 
mature and there are thousands of great tutorials online not to mention 
every host under the sun supports PHP.

But in the end, it all comes down to preference. If you're gonna host 
your own site and you already know ruby? Go for it!

-Chris