I agree that it is whatever gets you to the market quicker. However, if you are open source, you need to remember that you have to more or less move with the project as it involves. This is in good in that it will probably bring new and additional features. However, it also might create extra work when they change the "core" system.
1) If i program my startup it will take a little bit more time comparing to customize an existing solution. But it will take a little bit more time to add a new feature or make changes at existing features. I believe i will move slower because i don't know every feature and every line of code of an existing solution.
2) If i program my startup it will take a little bit more time. on the other hand i will know every line of code so i can move faster, can add new features in a shorter time, can make changes in a shorter time.
So my question still is there ...
Use whatever technology (language, library, CMS, etc) you can develope the product the fastest in and get it in the hands of potential customers.
You're going to rebuild the entire application at some point, so the framework doesn't matter at the beginning. Whatever makes you comfortable is the right choice.
I personally love building quick web apps on top of Wordpress because I know how to hack the system, it has a good library of plugins, and I program primarily in PHP.