I've been asked that during a hiring presentation before, and the answer I gave was that starting a startup was the most fulfilling thing I've ever done and you should definitely do your own if you have the inclination.

However, if you want to get a bit of experience working at somewhere where you can work on many systems at massive scale, want to jump a few levels ahead on the risk curve, or are interested in first seeing what a startup is like from the inside, then Justin.tv, Airbnb, Weebly, Scribd, Dropbox or any of the others are some pretty good places to do that.