PHP runs on most servers as an Apache module. That means you can get PHP cobbled into Apache with little effort and every Web host on the planet does this for you. Ruby does not run (well) as an Apache module. That means you have to manage the efficiencies of instantiating the Ruby interpreter as infrequently as possible. This is normally done by running Rails inside mongrel as a long running process.
The deployment difficulty is simply that of taking the mongrels down and pointing them at new code. Once you’ve done a few deployments, it will seem a lot easier. Oh, and Capistrano make it almost automatic for many cases.