4 or 5S in a D8 conversion?

[maneba]

Hi, I'm plannig to built a D8 conversion, but i'm not sure about the motor to buy.
I'll use the car in a 1/8 track.
The question is about performance (due to battery weight) and running time.
What do you suggest me between these two setups?

Neu 1512/3D F (1700 rpm/volt) with a 5S or a
Neu 1512/2.5 F (2050 rpm/volt) with a 4S?

1512/3D + 5S should be very energy conservative but i'll have more weight than a 4S.
Otherwise a 1512/2.5 + 4S should be a little energy hungry.
So?