Interesting, I always thought of it as the other way around, with the mk 1 and 2 being for long distance trips with lots of kerbals/cargo, the mk 4 being for medium range trips, and the ANT, being the smallest of them all, for short range mobility. The ANT is also the slowest, meaning that a long distance trip with one is going to take a very long time, and since it only has space for 2 kerbals, your time will not have done you much good. The RAT only has space for two as well, but it goes a lot faster, and of course the mk 1 and mk 2(crew config) have a metric crapton of seats, so even if the trip takes a while, you can get a lot of kerbals where they need to go.