woj was dead on... the benefits of RoR lay ONLY in the development... Better (read more traditional) OO support and the language construct differs quite a bit and allows for different ways to handle data manipulation. The RoR hype is spawned solely from developers...
It's slower to process server side, it's a HUGE pain in the ass to setup... PHP via Fast-cgi using nginx has been proven to be THE fastest setup you can get translating to less server costs on your end. Ruby does now have Passenger (mod_rails) so it's a bit easier to setup on Apache installations but I digress...
As woj stated you'll pay a premium for RoR developers because there are so few. This really comes into play if you're trying to get something done, even a simple change, and you can't find ANY...
PHP developers are a dime a dozen but good ones are VERY hard to find. However you're guaranteed to find someone to do cheap work to make a simple change so it's a much safer bet
