The externals are pretty darn easy to use, are cheaper, and don't require any modifications to the camera or remotes...maybe worth considering if you're thinking of a GPS toy. The Sony unit is just a little pocket fob that has its own battery, that simply needs to be synched with the camera clock. After that, no further contact needed (it can be used with any camera, btw) - it just matches the time in camera with the GPS unit time, and logs the photos. Gisteq is another brand with a similar device.
Or of course, if you've already got a GPS handheld mapping device like a Garmin, Magellan, etc...you can have your photos located by synching with that device.
Just a few other options for those thinking of trying photo tracking, so you can have neat map links on your photo galleries like me!

It sounds like the Nikon unit is much more automated, as it is installing the gps data direct to the photo exif when taken, saving having to synch it in the software later...and likely more accurate too, since it is precisely when the photo is taken, as opposed to the 10-15 second delay on the portable devices, and any synching errors that might occur.
It's fun though - I still get a kick out of clicking the map links and seeing the spot I was standing on a satellite overhead shot, down to the paving stone I was standing on!