I've always used CI. I know a lot of people tend to look down on it, but it's easy to use and suited my needs. Can't you install both Yii and CI and decide which one you like best?
If you want a full featured framework, Symfony2 is a good everything including the kitchen sink although it's a bit more Java-esque (which make sense since much of PHP's OO design is based on Java). http://symfony.com/
For something lighter weight, Silex is based on the Symfony2 components, so it's super easy to pull in more components as you need them. http://silex.sensiolabs.org/
Even if you're not using Symfony2 it's really modular and easy to integrate with other frameworks - we're running CI as our base framework due to legacy reasons, but all of our new work leans heavily on Symfony2 components.
I have used Cake PHP and Yii too. But they are not that modular and flexible when it comes to CI.