Zend Studio is more for programming than for design / cutup like Dreamweaver.
I use Zend Studio mainly for Zend Framework based projects (Magento, custom things, etc.); however, used to use it as a php editor without any ZF involvement.
Zend Studio is very good at PHP, HTML, XML, XHTML, CSS, and Javascript. It's very good at the web "languages". It provides you with what you need to create a very nice and rich website without getting in the way. Just because it doesn't have a WYSIWYG editor for layout, doesn't mean that it's bad or not for a particular purpose. Most designers will create a mock-up in Photoshop (or Illustrator) and then from there manually cut images, create a custom HTML document to make the cutup work for the web and then apply that to the pages, rather than letting Dreamweaver do the work fr you.
There are things that Dreamweaver does that CSS can do perfectly fine. Things like roll-over images. Dreamweaver adds a javascript class to do roll-overs when the css pseudo-class "hover" will work nicely. Same thing with the "active" psuedo-class (i.e. on click). But only certain elements can have hover and active pseudo classes applied.
If you really want to get into PHP programming, ditch the WYSIWYG editor. You shouldn't have to use any part of that for what you're doing in PHP.
Asking which is better is like asking whether Monterey Jack or Colby Jack cheese is better. They're both good, they both taste good, just one suits more people's tastes better than the other.
Personally, I left Dreamweaver back when it was still a Macromedia product and haven't looked back. I try to stay away from it unless a client says "I want a site that I can manage". Then I use dreamweaver templates since PHP is out of the question.
I'm not a huge fan of Zend Studio; however, it does serve its purpose. But so does Notepad++, Wordpad, EditPlus, TextMate, ViM, jEdit; or, any other text editor that's out there. It all depends on what you're looking to get out of the program.
If you want a WYSIWYG editor for PHP, then stick with Dreamweaver. I'm not sure why you'd want i as all you'll see is that little yellow code marker, but whatever. If you truly want to do nothing but code and force yourself off the WYSIWYG, find another program that doesn't have a WYSIWYG editor as part of it (Zend Studio, EditPlus, TextMate, Notepad++, UltraEdit, etc.).
If you want a review of all the different editors out there, look around the echo lounge for a topic about "favorite editor".