Got the iPad 16GB WiFi on the first day, but later ordered a 64GB 3G. After traveling with the WiFi only, I really missed using it for Maps, traffic. The 3G does unlock the iPad's full potential as some mentioned. The 3G version has GPS circuitry to allow faster GPS positioning by using the 3G towers. So, if you intend to use it for maps while traveling, the 3G is the better version. After a year, I've used the 3G data plan about half of the time.
I just upgraded to the new iPad2 64GB with 3G on AT&T - so my vote is still to get 3G version.