You could use something like Firebase for your API if your needs are not too complex, and then re-visit the back-end architecture decision once you've somewhat validated your MVP. https://www.firebase.com/docs/web/libraries/angular/quicksta...
PHP would make sense only if you have someone on your team that is already quite familiar with it and knows a number of PHP developers that you can hire to build out a team later on.
But Python developers with web development experience can be harder to find, depending on where you are geographically located. PHP web developers are everywhere, but the quality of talent varies greatly.
Now, to answer your question: pick whatever you're most comfortable with. Software startups fail not because they are built with PHP instead of NodeJS, but for entirely other reasons.
Use your energy to look for good developer / architect. Then let him/them choose their favorite technical stack.
Then choose between Javascript, Coffeescript or Typescript.
Just curious, are you planning on using TrueVault?
Good luck in 2015.