One thing i've always had to consider is what servers actually support Ruby. A few personal projects i've made in Ruby. However, when working on Web applications for other people (Who already have a server), they may not support Ruby (Or PHP for that matter).

I think it really comes down to preference. I learnt PHP first off, then discovered the power of Ruby.

I suggest taking a look at this: http://www.killersites.com/blog/2006/php-vs-ruby/