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?
The biggest problem I have with CI as a specific application framework is the insistance of being extremely backwards compatible (PHP 4 support was dropped less than a year ago, and they only very recently stopped supporting 5.1). I understand why this makes sense for Ellis Labs (the developers of it) since Expression Engine is designed to be deployed by clients. If you're picking a framework to use, pick one that allows you to take advantage of all the improvements made to the language (PHP 5.3/5.4 bring a huge number of improvements over even 5.2). I also have a number of fundamental disagreements with how CI is built - no proper autoloading system, a weak routing system (no URL helper/parameter mapping), no request/response objects, odd controller instation and library/model loading (generally fairly disconnected from standard OO design)
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.
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.