This may be too late, perhaps you have your answer already. Also I'm no expert so will humbly accept correction, but:
- Long battery wires can be bad as more inductance leads to more back emf and those spikes can shorten the life of the ESC. That's why there are big capacitors in parallel to the inputs from the battery and you can compensate for long battery wires by adding some more. There is a ton of advice out there, (low serial resistance, close to the ESC as poss etc.), but you can just buy little boards with appropriate caps on them, boards with natty holes and tabs such that you can get them really close to the ESC.
- Long motor cables (phase wires) are less of an issue and tend to just introduce more loss. There is a limit of course, you don't want massively long cables, but actually inductance on those ones is apparently slightly helpful. You may get less mileage as wasted energy heating the longer cables but performance and durability shouldn't suffer too much.
Those are generalizations, quite possibly wrong (really) and everything has it's limits. But I would err on shorter battery cables if you are forced to make a decision, and if the battery leads are still over 10cm then bolstering the source capacitance might be a good idea anyway especially with the giant currents an MTB is likely to draw. Caps are usually a lot cheaper and easier to get than new ESCs, (especially swanky VESCs) and having it break when out and about would suck.