Would you work for a promising start-up or an ailing investment bank?

pay at the start-up is a lot lower, but the director and largest principal has plans to get listed within 3 years. You'll be working directly with him and likely assume a relatively senior position in a short time.

The job entails making presentations to private equity firms, strategic analysis and private equity valuation.

The 2nd job is a junior level position in investment management, specifically equity analysis. very long hours, ultra competitive, but it's in a big name bank.
Job security really isn't there, but career prospects are fantastic if you can stay a few years.

Which would you choose?
You're 30, have a MBA, a CFA and spent the last couple of years unemployed.

Tough call. If you're a people person, go with the start up. If not, go with the bank. You won't make many friends there since its so competitive. If you thrive in that kind of pressure cooker enviornment go for it. If not, the start up might be better.