What is your experience when developing web applications using one of these two languages/frameworks? Which have you found to be more convenient and faster to develop with?
Either would be perfectly fine for developing web applications.
If your stronger in PHP than Ruby, then likely Cake would be "faster" because you wouldn't have to spend the extra time familiarising yourself with Ruby syntax. Obviously the converse is true.
I don't have a huge amount of experience of either but I'd say I prefer Ruby because I find the libraries easier to use and install (rvm/gems etc) and I like having a local development server rather than using Apache and the large and vocal open-source community on github/blogs/twitter is mostly a good thing.
Really, who cares, they're basically the same: Dynamic scripting language, MVC framework... If you're looking to expand your knowledge you might as well look at something very different like node.js or Haskell and Snap.
My take: if you are porting something with existing PHP code base, use Cake. For every other reason lean towards Rails. Better community and may I say more advance use of TDD, BDD, OO concepts (I just hated acronyms). Main reason is not the language's capabilities, but the mindset that exist in each's base of developers.