If you change your tyres and the rolling diameter is different, what difference will the location of the sensor make? A sensor on the front wheel will count revolutions of the wheel. One on the gearbox output will count revolutions of the propshaft, divide this by your diff ratio and you are back to wheel revolutions.
Also, if you go for the wheel sensor, you will need to fix a magnet (or similar) to each different wheel you are likely to use, which can bugger up your wheel balancing. (Unless the sensor is picking up from the hub) What benefits do you expect from a wheel sensor?