Ask HN: things to consider when negotiating with a startup?

Hello everyone, I just finished my PhD (about 2 years' worth of industrial experience) and I'm about to negotiate a job with a very small technology startup. Their field of activity is not exactly aligned with my PhD work, but there is some overlap. I like what they want to do and I think that it has lots of potential. I would be their first technical hire and this is my first time doing this so I have no real idea what to ask/look for. I also have a job offer from a big company (a stable job, but with less potential for long-term earnings). Basically it's the whole risk-reward trade-off and I don't have a strategy of approaching it. How should I proceed? Could I ask for a percentage? If yes then how could I know how much I can ask for? What specific details should I pay attention to? Thanks!