Sounds to me that you are interesting in joining the startup more than the large company. What on your mind is that you try to rationalize and compare the stability the large company offering against the possibility of success of the startup.
Since you are young (I just assume so, I'm 24) and have few dependencies and debts, if I was in your shoes, I'd decide to work where my passions would want to work. With the startup, you can make a much bigger impacts and have more freedom to create and innovate. Bigger company provide you a more stable life, but do you see yourself working there for 1, 2, 3 years?
My thinking is that you can always get a job with a traditional company. It's a matter of putting together a solid resume, working through the network of friends and recruiters to place yourself in the right place. On the other hand, joining a start up may offer you the chance to do something different. In a way, you can really learn a lot from working your way at the start up, wearing multiple hats -- these should be invaluable experience that you can brag about in your resume for future employees should you apply for a new job. Best of all, you still get paid working for this start up.
Here's my quick list to compare the pros and cons between the large company job and the startup job:
Large Company:
- you're a number, not a name
- what is the chance of you being able to do something creative?
- how difference the salary would be, compared to the startup
- benefits: insurance and dental are important, if you need insurance, then I'd do some research to see if you can get adequate coverage if you work for the startup company.
- 401K: nice to have, but don't let it determine your life now when you can follow your passion and do something meaningful
- you probably can get a job at a company anytime when the economy recovers.
Startup
- you're their 4th employee. 25% of the workforce.
- equivalent salary. Based on what you said, this should be enough to get your house.
- benefits, retirement: is there a chance for stock options? For retirement, how about starting a Roth-IRA for yourself (you should do this anyway regardless)?
- health and dental insurance: you can do some research so you won't get caught in emergencies without proper coverage, especially when you plan to have kids in the near future. If you're wife can get you on her insurance plan, then it's one less thing to worry about.
- sounds like your passion is here. You do want to work for the startup. Ultimately, what's your gut telling you?
Good luck!