Boost or buck regulator for battery life?
In general, and all other things being more-or-less equal, I2R losses are going to be the chief cost of efficiency in a power converter. Therefore, it would be a goal to keep currents as low as possible.

This would imply that the overall system efficiency will be better with a buck converter than with a boost converter. The average currents in the batteries, coil and the switching element will all be lower.