I like to start exploring the maps with the 4x4 and thats the only job that the 4x4 have (until the crane trailer, huh? huh?). The problem is that the 6x6 are simply better at that. Take any empty 6x6 (not the 131 LOL) and it just plows through everything very fast, so fast that it feels almost like cheating. So fast that makes you think that the 4x4 have effectively no role in this game.
You could say that the 6x6 consume more fuel but 200 L is A LOT in an empty truck and in case of need you can just go to a fuel station near to an exploration point. The time you loose doing that is minimal and in exchange you get a much more enjoyable and less frustrating trip.