I am in the process of migrating to a new PHP Framework. I have been involved in heavy development using the CodeIgniter Framework, but I am finding it a little too lightweight for my needs now. 

I have boiled my choices down to either Zend Framework or Symfony. I know the learning curve for both is relatively high. However, I just wanted to get a rough idea of which will be worth getting stuck into (as I will be spending quite a lot of time getting familiar with the chosen framework). 

If it helps narrow the answer down a little. I am not looking to build a simple blog or anything like that. I am in need for something that will support full-fledged development of e-commerce systems, customer relationship managers and content management systems.