Originally Posted by mark1976
Are there any good CMS framework written using Ruby ?
was written with Ruby. Haven't used it myself, but I've heard good things about it.
To answer the original question, Ruby and PHP are used for more or less the same kind of programming, but Ruby is faster to develop with. I'd recommend doing a few basic tutorials for both languages and sticking with the one you find more interesting.
Also bear in mind I'm answering this question as a Rubyist, so I may be a bit biased.