A few details
GPS have refresh rates, typically, not always, the pricier have a higher refresh rate.... We own a Sailboat and there's a big difference between, say Lowrance, Garmin, and Raymarine (Raytheon). Go their websites and research it.
GPS need to "see the sky" They won't work in caves.Sometimes, there's an impact by a heavy leaf canopy. If you mark a spot in the Summer, verify before you go back in the dark.
Some have more bells and whistles than others. By that I mean you can interface with you PC and save configurations and build "lists of waypoints by category. Some have a red backlight to help preserve night vision.
In the past, the Government has imposed WAAS (Wide Area Augmentation System) limitations. They dis this during Desert Storm. With WAAS, which was established for Airflights, typically, you can expect an error of 1 meter laterally, and 1.5 meters vertically. Several things effect this again good reception=good results.
DGPS (Diffrential GPS, NDGPS in US) uses Ground based transmitters to increase the accuracy for GPS. It started out for Mariners being controlled by USCG. Now Homeland Security Controls it and much of CONUS is covered by it.
Lastly, my opinion, in a boat, especially on navigable waters, the high end Garmins and Raymarines are hard to beat. But, getting into a narrow approach in heavy fog at night, in a strange location DGPS+Radar+good charts+good depth instrumentation= reduced pucker factor by about 50%.
On land the Garmin are hard to beat. The downside is the Mapping GPS may require the purchase of new mapping software. The upside is that this software usually employs both Topo, road maps and sometime satellite views! Garmin's site is very informative without overload. I hope this helps. Oh, I do own one for land use. I use the GPSMAP 62. I like the maps and the satellite view (like Google Earth).