There is an excellant article in the May 2010 issue of Fire Apparatus & Emergency Equipment on page 18. It points out about the extra wear and tear on the apparatus over years of service with a single stage pump over comes the upfront cost of the two stage pump. I have been servicing fire apparatus since 1978 and agree with the information the article points out.
We bought a Draft Commander for pump service testing. Using the thermometer on the Draft Commander at 60 degress, it took 4 single stage pumps to heat the water to 95 degrees ( 4 hours), while it took 6 two-stage pumps to heat up the water to the same temperature (6 hours). All the pumps were rated at 1500 gpm. These figures are based on a year of testing results.