Pros:
- Possibly more educational
- More face time with executives
- Likely to be less rigid in structure, responsibilities, working style, etc.
- Possibility to share in profits if company succeeds
- Possibility to earn valuable stock options (though you should always treat stock as worth nothing if the company isn't cash-flow positive or doesn't have a thriving customer base)
Cons:
- Less of a resume booster (in the eyes of some)
- Potentially worse salary
- Potentially more/harder work
- Less opportunity to switch roles, offices, teams, etc.