Startup building and mokup
The idea that PHP/MySQL is easier or simpler than say Python/MongoDB is just inconsistent. 
If you compare for example, Django (the most popular python web framework) with symfony(PHP) you will find that they are almost identical in terms of features and architecture (symfony is actually slightly more complex but also has more very advanced features).
For mockups, if I were you, I would use solely HTML/jQuery/CSS. 
Build your pages just like you would like to have them in your beta version, use jQuery to load sample data written in json. 
That's all you need. You can even find WYSIWYG application to speed up the process.
Later on, you can build the back-end application using either python or php, it won't matter. 
The integration process will be identical, create your models, create the controllers, and use the HTML you already have as templates.
Building your app in php/mysql then convert it to python/mangodb will make you rewrite almost all the code simply because python is so much different from php (easier I would say too, but that's just my opinion) and because mangodb is not a relational database meaning you will have also to rethink partially your architecture.
since about two years ago, I did find my interest in code (Hardware/Sytems/Web) and now, I've found a project which motivates me a lot (It takes all my free time indeed).
Starting this point and because my project could soon switch from a free time project to a daily job, I'm currently developing a mockup of this project based on PHP/MySQL and JQuery.
Even if I'm a true Python/MongoDB lover and a System Engineer, I did prefer those technologies to build up my mockup because of their simplicity to build a complete functional private stack at home.
I'm pretty advanced on my mockup and it seems to work as I want it.
Now I'm wondering if, about your point of view, would have been better to start to build my mockup using directly the targeted technologies (Python/MongoDB) rather than to use the easy PHP/MySQL couple to do it?
Obviously, because I plan to made this project my daily job, I had to have something visually functionnal to be able to raise a little bit of money, and about me, using an easier stack it's more easy, but I would like to have your feedback on this kind of question.