Apple MacBook Pro Battery vs Processor
performance per watt
You want to know the 'performance per watt'. In computing, performance per watt is a measure of the energy efficiency of a particular computer architecture or computer hardware. Literally, it measures the rate of computation that can be delivered by a computer for every watt of power consumed.
2.5GHz Quad-core Intel Core i7 = 21.05 pt/W
2.8GHz Quad-core Intel Core i7 = 33.4 pt/W
So, when only looking at the 'performance per watt' you would be better of with the 2.8GHz
Sources used: Wikipedia, cpuboss.com - 2.5GHz, cpuboss.com - 2.8GHz
note: not 100% sure if I picked the correct processor types.