Series vs Parallel Wiring

[Serum]

Assuming that you are sticking with 14 cells in total,

parallel. the truck won't move as fast, because the motor makes a certain amount of rotations per volt. Serial makes the volts higher, parallel makes the capacity higher.

if you use 4X 7 packs (2 serial, 2 parallel) you will get better runtime and the batteries can deliver more power. (twice the power to be exact) But the weight is what kills all the positive things. (and most likely, your drivetrain)

If you run the packs parallel, you need a motor that has got a higher KV rating (higher RPM per volt) and a controller that can handle more current. because to get 800 watts of output energy the basic math would look like this on 14 cells;
16.8VX47A=800 Watts

on 7 cells that would be;
8.4VX94A=800 Watts

And in most cases, the higher the voltage a setup runs on, the more efficient it will be.

So a 2S 7pack will probably run longer than a 2P 7 pack.