What's the point of being rich if you don't have someone to share it with? Money alone isn't going to make you feel successful or happy. If you think this girl might make you happy, I'd take her over the start up.
Business partnerships are already volatile enough without adding romance into the mix.
Ever hear of the mom-and-pop store? Couples, even married couples, have been running businesses together for centuries. My parents owned and ran businesses together. In fact, "can I run a small business with this person" isn't a bad question to ask yourself about a potential life partner :)
My parents owned and ran two businesses together, but they never got rich. But it wasn't a bad way to live, and they definitely saw more of each other than they would have with day jobs. Though to be fair, they also had my dad's retired pay from the military, which mitigated a lot of risks and meant they'd never be truly broke.