If you work at a large company you need to work for a longer time to have enough cash, relevant knowledge of wide business area (not just something highly focused), and it is a safer choice.
However if you are one of those who like to see change, cannot stand lazy co workers, and are excited about immediate impact of your actions then startups are the place to be. It is riskier, more fun, you get enormous experience in a wide domain which might not directly be related to your work. Whatever be the reason, please do not work in a startup because you think it will do well and you will be a millionaire because of it.
Also regarding: If you are not a cofounder, you will be expected do a lot of different types of work, affording you little time to spend on the particular subfield of interest. I for one know a lot of co-founders who work their asses off and party alongside their employees. This is in my opinion the best part about working for a startup that you will never find in a large organisation. There is no-one who is a "boss". You are a bunch of people who have a passion to make an impact and everyone is focused to make it happen.