As the name tells you a release candidate is never ready for production. But the release of Bootstrap 3 seems to be planned on august 19th 2013 (see: http://blog.getbootstrap.com/).
The mobile first strategy is no argument to choose cause TB3 also has this now.
Make a list of differences between both frameworks and compare these with your wishes and capabilities.
Zurb's Foundation / Twitter's Bootstrap 3
MIT Open Source License / Apache 2.0 license
Sass (SCSS) / Less
Foundation-icons / Glyphicons
Javascript components / Javascript Plugins
etc.
Good luck
PS Hi @Draconar i can't choose for you, but when i was you is should start my project with TB3RC1. Cause this is a RC i don't expect major changes for the final release. As you mentioned TB has a big helpful community so i don't expect you will hit a dead end ever.
But when you don't want to take this risk i should prefer ZF above Twitter's Bootstrap 2.
Twitter's Bootstrap 3 is not backward compatible with Twitter's Bootstrap 2 so migration will always hurts you.
There will be one moment you will need a new feature which TB2 not have or support always in future.
I also so have to tell you i don't know ZF very well, so i can't tell you much about their community or support.
I will advice you to read this too: Use Zurb Foundation 4 / Twitter Bootstrap 3 For Non Mobile First Projects
