Alain -Both buses have a GVWR of 14,515 kg. As stripped shells, there's probably 907 kg difference in their weights, or roughly the weight of 13 passengers, obviously the 40-footer being heavier.
Fuel mileage difference between the two with an automatic might be 117.6 L/100km at the most. Figure about 33.6 L/100km for planning purposes and you'd be darned close for either bus. With manual boxes, the '06 will squeak out 26.1 L/100km and the Buffalo 31.4 L/100km when speeds are kept around 100 kph. The 4106 has a 530L tank, the 4905 comes with 625L, so a little more range.
More interior space and HUGE baggage bins are a real plus for the Buffalo, vs the nimbleness of the '06.
FWIW & HTH. . .
