You have the potential of having a major impact on the company. From a financial perspective, an established company can usually survive extended periods of losses where a startup might go under. There is nothing worse that having a payroll check bounce. Established companies are usually more stable than startups. A lot of people have worked long hours for startups, only to have them go bankrupt.
Startup companies take a lot of energy and go through numerous changes.
Rapid changes have a tendency to burn out some people and frustrate others. So by working in the established company, you might avoid the sense of urgency and change that exists in a startup. Benefits packages, such as health care and holidays, are usually better at established companies since they enjoy economies of scale. Insurance packages are usually better, too, and you sometimes have more choices, offering the potential of a better fit for you and your family. Individual mistakes can be tolerated more in the large company and might not have a major effect on their business model.