In general, higher voltage will be more efficient as losses are relative to the current, and power is current times voltage. If you use a the same (or better - a higher) Ah capacity of the batteries you will have less voltage drop, and increase your range. Basically, if you drain lead-acid slower, you will get more energy than if you drain them fast. I guess you have already made sure the car can handle the higher voltage.
Lithium is Expensive, but better!
Lithium is Expensive, but better!