I really prefer the 3G one. It has, obviously, 3G functionality which is nice when you need internet, but it also has GPS. Now this might seem useless, but some apps make use of the GPS functionality and it's actually rather handy.
And when I went on holiday in december I brought my iPad with me for use during the trip and perhaps on holiday, I thought. But at some point during the trip the TomTom couldn't find any GPS signal due to bad weather, in the middle of nowhere (in Germany 😛) and when it found a GPS signal it couldn't calculate any good tracks because all roads were routed (or however that's called in English).
Well, my iPad on the otherhand had a good GPS signal and we were able to find the right track by using the built-in Maps app in combination with the GPS! It's a situation you think will never happen, but it happened and thanks to the big screen we were able to give clear directions to the driver.