I would decide depending on the complexity of the requirement.
If it's a very simple I don't see a huge difference in using a framework or plain old php. But if it's a complex project, I would definitely go with a framework considering the fact that most of the core functions one might need are already there and quicker development. Again, it would come down to personal preference too.
Codeigniter has been said easy to learn for beginners in framework. Symfony and zend are more commercially used (I am not saying others are not, but I see more vacancies asking for symfony or zend development experience).