Piston water pumps - why are they better?

Am thinking of going to on demand system, and changing from water storage tank and RV pump to boost pressure. ONe of the posts here covered a number of topics and included an interesting discussion of piston pumps - which I gather are different from displacement (shallow well and maybe some deep well?) and centrifugal pumps (Grundfos eg). It was mentioned that a piston pump is really good in that is uses little power and works well, but that were hard to find. Anyway, I found this link http://earthmovement.net/FMC_Bean_Pumps/FMC_Piston_Pumps/duplex.html which covers a lot of different pumps, including piston pumps and wonder why they would draw less power and whether they are really worth considering in a solar system like mine? Basically I have a battery bank and panels on the roof and an outback 3524 but set for weekend use primarily as it is a cottage.