The question has been asked over a year ago, so I guess you've already made up your decision. But I will answer it in a general way in case someone else is having trouble making up their decision for their own project.
For my current project I'm also having a hard time deciding whether to go for the Zend Framework, CakePHP or Symphony.
But I think I'm going to go for the Zend Framework for these reasons:
1. Learning Objectives
I know with CakePHP you can get started quicker, but with a blackbox framework such as that I feel I'd be missing out on a lot of learning. If I used CakePHP I'd be missing out on learning about OO concepts and patterns because I just won't get the chance to apply them. And, if I were to stop my freelancing work and look for a job I want to be able to tell me employer I really understand OO programming, as it's in pretty high demand now. A common argument is to go for CakePHP if you're new to programming, but I strongly disagree. When you're new you want to understand the concepts behind all these things first, before you start using a framework that hides implementation details from you. And when things do break in a blackbox framework such as CakePHP you want to know exactly what's going on in the background so you can fix it. It's kind of like using Dreamweaver but not understanding HTML! You need to understand the architecture of CakePHP, and for that to happen you need to know the OO concepts, patterns and architecture behind it.
A bit of learning goes a long way. It might be harder in the beginning when you need to write more code by yourself, but in the long run it's a lot more enjoyable to understand what is actually going on. There is dogma that says "Don't write anything yourself that has been written before". OK, I agree mostly, but if you have much to gain from writing something yourself, it's better to do it. The things to gain may be having a more flexible framework at your disposable, and of course better understanding of what you're doing.
2. Documentation and Support
What if the Framework developers decide in a flash of brilliance that no one needs documentation for version XXX of the framework and overwrite it with a new documentation version, but you are still using that old version? I'd say you're screwed in a big way. The guys at ZF put all the doc versions at your disposal -- http://framework.zend.com/docs/api and it's something you'd just expect. I strongly believe that a framework is only as good as it's documentation.
If you're going to create a very long running project with lots of unknowns, I'd go with the Zend Framework. It's got corporate backing and official training courses. CakePHP does have a great community, but there's always more security with corporate backing. The fact that Zend is commercializing it's products (the Zend Framework is open source), does not bother me in the least, and is more or less irrelevant. I couldn't care less about ideologies on free software or whatever, I just want to get things done.
3. Flexibility
You never know what the client will ask for next (or what you yourself will need next). If he/she asks for some insane/strange feature from Mars that will break or stretch a framework beyond its capabilities, then you're going to be in trouble. The Zend Framework is well known for being very flexible - you can pick the components you want to use and it doesn't force an architectural doctrine on you. In some cases you could go for an existing CMS, but CMSs are only good for websites that fit a certain mold, beyond that they're no good.
4. Project Hand-off to Other Developers
If it ever comes to this, you want to make sure the clients have access to a large number of developers who understand the framework you have used. I don't know the numbers here so I can't honestly say whether Zend or CakePHP has a larger developer base. My guess is Zend as it has a lot more financial backing than CakePHP and the organization has the ability to officially train developers who use it. Probably it's easier to hire a Zend developer by asking for his/her certification.
5. Continual Development of the Framework
Is the framework continually developed and at what pace? Is the documentation able to keep up with the framework development? And, does the organization support older versions of the framework? What if the lead developer of a framework gets hit by a truck? What's going to happen now? Is the organization that develops the framework as brilliant managerially as they are technically? I give Zend Framework the go here.