With barracuda... My semi-educated guess is the 4.8 pack will deliver more flight time. But the diff will be very small.

My logic...

The 4.8 pack will start at a voltage higher than 5.0V, but will quickly drop below that as it hits it's nominal voltage of 4.8V. Hence, it will use less current than a constant 5V source over the course of the discharge cycle. In addition, there will be some losses due to the regulator on the 6V/5V regulated setup.