It seems to depend on the size of vehicle and where they put the engine and gearbox. Smaller tracked AFVs, CVRT, FV430, M113 etc have the drive sprocket at the front. Heavier AFVs, Chieftain, Challenger 1/2, M1 Abrams etc have the sprocket at the rear. I guess this just depends on where the steering box or gearbox is. The lighter vehicles have it at the front, heavier at the rear. AS90 and Warrior are heavy vehicles, with the box at the front as space is needed at the rear, so the sprocket is at the front. All the vehicles have track tensioners.
Chris