GPS units, by default, write out a set of sentences once a second, but this default can be changed. Perhaps the Sparkfun unit would be less power hungry if you reduced the number of NMEA sentences to the minimum that you need, then reduce the rate at which the devices transmits sentences?
Having said that, I would assume the GPS receiver itself is probably the biggest power hog in the system. The Garmin GPS18 is 50 to 65mA (depending on which model you choose).
-j