This is a comparison chart of what APRS functions each GPS has.
NMEA 0183 is a standard that send serial ASCII data (usually at 4800 baud) with information of your londitude, Latitude, speed and a lot more.
Some GPS units allow NMEA data to be sent to them, this often is display on the screen as waypoints.
Does the GPS unit support NMEA 1.5 as early Kenwood TH-D7 need it.
Does the GPS unit have built in maps.
Can you upload new maps to the unit, a good source of free maps for garmins is Velo maps.
How much space is availble to store maps.
How many stations can the unit support.
Does the unit support Kenwoods station icons.