From the www.amazon.com page and the Garmin page for the GPS 18 receiver:
The GPS 18 is offered in three different cable configurations: A-style USB, DB-9 pin serial with 12-volt cigarette lighter adapter (PC), or bare wire (LVC). The PC and LVC versions both default to output data in the industry standard NMEA 0183 data format, but may also be user programmed to output data in the GARMIN proprietary format. The USB version produces data only in the GARMIN proprietary format. All three versions of the GPS 18 come complete with non-volatile memory for storage of configuration information, a real-time clock, and raw measurement output data for sophisticated customer applications. The LVC version additionally provides a pulse-per-second logic level output whose rising edge is aligned to the UTC second within 1 microsecond. The USB 2.0 full-speed version of the GPS18 is also compatible with USB 1.1 full-speed hosts.
It looks like the USB version will not output NMEA strings: "The USB version produces data only in the GARMIN proprietary format." I consider this to be a reason not to get the USB version. Otherwise, it looks like a good receiver.