For people that know they travel plenty then the 3G kindle is the better choice. They use the cell phone connection using the AT&T towers within the US (both 3G and the regular slower cell phone if 3G is not available), and partner with various cell phone providers outside of the US.
Kindle coverage is shown here:
http://client0.cellmaps.com/viewer.html?cov=1
You can zoom out and move around the map to see 3G and cell coverage (based on the AT&T and partner cell phone towers.