I'm exactly in the same position. In a startup situation, you should take the job if you think they company can grow and be profitable. But because you are helping to start the company and lead it to success, it is common to ask for a shares or equity, so if the company grows, you grow with it. That will mean you can potentially earn MUCH more than just having a salary.
So you should decide based on:
Do you like the startup and their project?
Do they have the potential of succeeding and creating a profitable business?
If so, take the job. You can ask for a little less of market salary in exchange of shares.