I've worked at a growing startup and, currently, I'm at a medium sized corporate. The problem with the startup is that you get a lot of responsabilities and sometimes can't focus at just programming, because you are helping with client support, or helping the sales team, organizing events, etc. That can be really overwhelming. On the other hand you can expect to grow really fast and earn a lot better as the company grows. It's like a risky investment: you either get a lot of profit or lose it all.
One risk you take it's that you don't have a well defined carrer path, so, when the company achieve stability don't expect to grow any further.
I guess where to work really depends what are your short term and medium term objectivities. Currently I am seeking stability and work only 8h/day (at the Startup, at least here in Brazil, you have to work, in general, 12h/day), so I think working at a medium sized corporate is being the best thing to me