In my opinion, it is always better to join a startup as your SECOND company (or later).
One should start off by working in a big company (corporate) first, as it helps you gain a broader understanding of how a company is run - corporate culture, processes, and a sense of hierarchy.
It is also a good place to meet some very good connections.
Once you've absorbed enough over there (and feel bored enough to want to leave), then by all means try out startup life.
When entering a startup, fundamentally the most important things are:
- Who will lead you.
- What you'll learn.
- Who you'll meet.
And for yourself on a personal scale, you should be clear on:
- Where you'd like to be in 5 years
- What you love doing and want to keep doing for the next few years
- Who you enjoy spending time with (or want to have around you)
Everything else is secondary.
Hence, formulate your questions around these areas listed above, and you will never go wrong.