I'm currently looking at kohana - I use Zend FW now (been doing a long time without any of the established frameworks, only my own set of tools). Can you elaborate a bit why Kohana instead of Zend? Even if I end up using Kohana, there are several things in Zend I will keep - PDF for example.

I don't have any experience with Zend, but I've heard its just a very heavy weight install.

Kohana is super light and easily extended, though you lose a lot of the fluffy libraries.