We believe that a start up increases career equity more than a larger company. Value is somewhat based on perception, and the perception in the market is that only the top 20 percentile quality people can work at a start up.
Data Points:
Working with people better than you brings out your best.
Which reference do you think will be more valuable? Paul Buchheit or Joe Blow from Acme Big Company?
You get more experience and have more accountability at a start up, which translates to more accomplishments.
Start ups are typically ahead of the curve with respect to up to date tools and technology. The more leading edge your skill set the less people to compete with and therefore more opportunities.
Bottom Line: If you are motivated to get up in the morning and get to work on your project, you are in a good situation. But for maximum career value, work in a start up.