There is advantages and disadvantages to both.
Extending the ESC-motor wires will produce a lot more electrical noise, while extending the battery-ESC wires will put higher strain on the esc capacitors.
Personally I think extending the batterywires is better, as noise from the motor has downed planes for me before, even on expensive FM systems.
On 2.4 GHz, I believe the problem is virtually nonexistent.