Neither is better than the other. While one may be more suited to particular applications than the other, they are certainly both good frameworks.
PHP is said to be faster, Ruby is objectively a better designed language. These probably make more of a difference than the framework itself - you can always modify the libraries or write your own classes to make the framework do what you want it to.
My advice is to stick with what you know if you are happy with it - learning a new framework is a long process. If you have issues with Cake or you're keen to try something different, Rails is definitely worthwhile.