Also, it's not really certain that you'll learn more at a startup than at an established company:
- Most startups fail, so all you might end up learning is one particular way how not to run a startup, not how to run a successful startup.
- Many startups are run by younger people who themselves may not have had very much experience shipping products or running a company.
- Startups are under pressure to deliver quickly before the money runs out, so you might not spend as much time considering alternative approaches to problems, which is one of the ways to get deep understanding of a problem domain. The desire to move quickly might also pull them in the direction of quick and dirty solutions instead of solid engineering.
On the plus side, you might end up with more responsibility for the product than you would get in a larger company, and there are many things you could learn from that experience.