As I understand it the helical rotor is best for high head/pressure and low volumes, centrifugal better at lower head and high volumes. I'm not sure where the crossover point is.

Centrifugal can handle more foreign matter in the water which may be important for you pumping from a stream. Depends on your water quality and how high you need to pump but I think you are right that the centrifugal would last longer. 

I have one of each pumping from bores, already installed when I bought here so not sure why one of each, they both pump about the same height. The centrifugal has gone at least 7 years no issues at all. I have had to replace the helical rotor pump after it jammed, pump shop said the rubber stator had swelled and jammed the rotor. I dont know how old it was or if its a common problem. Both just keep running every day the sun comes up. (I had never run a solar pump until these ones so I'm far from an expert on pumps, but they sure beat a windmill).

I would ask your supplier why they recommend the helical rotor over the other, but I agree with your thinking.