I really like cakePHP but alot depends on what experience you have and what you are trying to build. Will there be a greater number of plugins that already exist to speed up your development? Will one fit in with your existing programming skills better? I'd download both, install and work through a tutorial to see if one feels more intuitive to you.
I have recently chosen Yii over any other as its features are too good. Natural Programming style supported by it is best! Complete OOP. Have good and rapid growing community and many plugins / extensions. Yii is ideal for Web development. No doubt, there are many others which are good, but that again depends on personal preference at times. You can find nice comparison about frameworks on Wiki Page.
I agree with Sarah: You need to look at both, and decide which one is for you. Luckily, both have excellent documentation. (CI has lower learning curve, and you could learn it faster.. but thats just one of the arguments.)