I have not done this but it would be interesting to calculate the teeth mesh area of one of the spur gear PSRU and then compare it to the area of the six pinion sun and planetary gear area engaged to transfer a similar amount of power.  I could be wrong, but my gut feel is that the sun and planetary probably have more metal to metal contact area for transferring power than the Sun gear.  But, like I said - just a hunch, would be interesting to know.  Anybody have a quick answer??


 
Ed