A big difference is in how models are created: CakePHP models are written in PHP, and Symfony models are written in YAML and powered by Propel. CakePHP's approach is more similar to ROR's ActiveRecord (although it isn't exactly an AR implementation). CakePHP, in general, is more rails-esque.
CakePHP's documentation and tools, in my opinion, have a wider target audience and the syntax and helpers are easier, but thy have yet to embrace PHP5 as their exclusive target (to autoloading isn't really there). In general, I prefer CakePHP's approach because it sort of follows an established standard, and I applaude it's organization. I'd also recommend Kohana for it's PHP5 goodness.
There's another post on stack overflow about this question, although its a bit different in focus.
Edit: I revisted Symfony to find the reasons I said 'no' and came up with these — your opinions and mileage may vary:
CakePHP also offers dead simple scaffolding and easy to understand CLI tools. Symfony's CLI syntax is a bit wonky to me, and 'CRUD' in Symfony just isn't the same. Combine that with Symfony's (awkard) action syntax and throw in Symfony's poorly designed (and challenging to understand) website, and preference for 3rd-party paid documentation (books on Amazon) and you have more ticks in the cons column.
