Bergen to Stavanger: Bus, Boat or ?

If cost isn't an issue I would choose the boat over a bus any day! The ocean and shoreline in this part of Norway are stunning...I did this route a number of years back on a hydrofoil and it was beautiful and loads of fun. I heard that the hydrofoil service was stopped a few years ago but am not sure if that information is current...you would need to check.
There is nothing wrong with the bus and it's cheaper - but it will be slower too.

The boat service was stopped December 2013.

Also look into flying. Several airlines fly that route and tickets seem cheap enough. Look at Skyscanner to see all options: http://www.skyscanner.com

I took the bus four years ago on a reverse of the trip and it was lovely! Went on several ferries, as well!