The difference between the internal antenna and external antenna is where to install. Internal antenna is build into the gps device. External antenna will be installed in  a outside  location.
Gps internal antenna can accept the signal through the windshield but not through thick and hard thing to accept signals

Some devices built built-in high sensitivity GPS antenna. These devices may be able to pick up the position of the GPS signal
 

The best way to determine whether the need for an external antenna, you can install the device on which the location and power supplies. Allow plenty of time to get the device GPS signals. If it can receive a signal at that location, no external antenna. If the device can not get a GPS signal, for so long the need for external antennas