Background: I did a 100k+ users web app using cakePHP and was very happy with it (2 years ago). Now a day, I use Django/Python as my main weapon of choice when I am thinking about complex web application. For simpler sites/CMS, I'm actually giving wordpress a try.

My thoughts on your question: I am a developer first, so I tend to forget the most important part of the business-have one! If your product isn't launched, you don't have a business just yet. Therefore, if using cakePHP can get your stuff out faster and with a simpler server setup, go for it. If you're comfortable with both (Django & CakePHP), then choose Python. It's easier to fix a painting when you make the first stroke. It's that blank canvas that scares us all. Go ahead, make the first stroke. Do the prototype in PHP if you can do it faster. Once the logic is written in one language, it doesn't take that long to redo it in another language. At least that's how my experience had been.

Good luck and hope you resolve your disagreement soon. There's no time to waste.