Durability and strength mainly. And you could probably make it a bit thinner in Alu/Ti so the magnet could be closer to the sensor.
jd_3d have you thought about keeping the gears nylon/plastic and putting them in a machined aluminium body. I'm just concerned about rocks/sticks etc destroying...
I'd Pay $300 for an Aluminium one. Thought of doing a Kickstarter to get funds to increase the volume to reduce the per unit price? There would have to be a market for it.