Cake is laid out much like Rails and obviously takes a lot of inspiration & ideas from it. Cake is a nice introduction to MVC frameworks and rails seems pretty straightforward coming from cake experience.
Ajax is super easy with Cake using the JS helper. In fact everything is super easy. Its a great framework, especially for distributed apps (eg cms's) or any other situation where the ease of hosting a php app is a benefit.
I would see the main advantages of rails being Ruby (and therefore the better OO implementation of rails etc) and the community. Gems (much fewer / less comprehensive cake plugins), training materials online, books (eloquent ruby anyone?) meetup groups etc.
