Just found a power point which covers a lot of the research done comparing ponds vs bioreactors. (see: The World Congress on Industrial Biotechnology & Bioprocessing
For small scale systems, bioreactors are better - but for large scale production - there is not much difference except for cost. A lot of commercial bioreactor "farms" shut down or end up rarely being used. One was setup at a cost of more than $1 million and closed after 2 weeks.