[asid61]

I haven't heard of the Vex shaft wearing down except briefly in 2014. They have improved it since then so that it no longer wears.
As stated above the encoder attachment is nice. If you have a lathe sticking an encoder on the WCP one is quite easy either by modifying the gearbox upper shaft or on the drive axles. I would just go with whatever is cheap (assuming you can get over the encoder stuff).