well, the long and short of it is "either", "neither" or "both" depending
The front one tends to get a lot more "head on" collisions, such as driving into rocks and so on, although in a series motor you're likely to do a lot of damage to the steering linkage before you reach the diff.
The rear diff tends to be subject to you balancing the weight of the entire motor on it, and then dragging it over a rock for good measure...
in the later case, you're more likely to have ripped the gearbox (or at least the gearbox cross member) to shreds before reaching the diff...
so as I said, depends how seriously you're thinking of playing, with the cost of series diff's being so low, and given that you can always drive home on one of them (with judicious removal of a prop shaft) you might want to save your money ;)
