If you can afford to, get as close to the action as possible (I take it in this case it would be to join a startup).

In my personal opinion, startups aren't really a viable business strategy. The general idea of a startup is to attract investors to your idea with no reasonable timeframe regarding profitability, instead focusing on fast tracking the creation of a product/s. I really prefer the small business approach of prototyping and reworking to reach profitability and with every action taken to keep in mind the most important question: how will this directly make me money?