My problem with PHP Form is that it doesn't compare when you talk about the styling you can do with CSS Form Builder. I have used both... I'm not sure that the type of user who would use CSS Form Builder wants anything to do with gray background table based forms when you can style the forms in CSS Form Builder to look like your site, save any of your styles as templates, and be ultimately more efficient as a designer.
One is also a Dreamweaver extension and the other one is not. I like the fact that when I create my form with CFB, I have the option to, right away, add in other WebAssist functionality whether it be DataAssist or Universal Email -- or -- any other extension in my arsenal for that matter.