I have had many conversions with people about the differences in working for a startup and a well established company.    The following quotation sums it up well:

Each morning the gazelle wakes up knowing it must run even faster than the fastest lion to survive. Each morning the lion wakes up knowing it must run faster than the slowest gazelle or it will starve to death. Regardless of whether you are a lion or a gazelle, when the sun comes up you better start running.

At a startup, it is simple, you need to run hard as hell in order to service whether or not you are a lion or a gazelle.  If you do not, then you will either be eaten by the lion or starve as the lion. 

At a well established company, there are other considerations that fall into place like whether your action will upset the existing business.  As a result, you will be unable to run as fast as you can so the chances of long term survival are diminished.

So when you choose between these types of companies, think about your how your personality fits into each of these environments.