Ford super duty 3.31 rear end. Will it be a big mistake. ?

^^^^^ If you need more torque climbing a grade the transmission will drop a gear. Say you are pulling in some hilly terrain. With the 3.31 equipped truck you might feel 10 gears is too many so you lock out tenth. Now the 3.31 is in ninth running almost identical rpms to the 3.55 equipped truck. You come to a hill and both trucks require full throttle. Both trucks are going to drop a gear at the same time. At 60 mph the 3.55 will drop to ninth and the engine will be at 1446 rpm. The 3.31 will drop to 8th and be running at 1668 rpm. If pulling the hill requires more mower than the engine can deliver at 1446 rpm the 3.55 will downshift to 8th. The 3.31 may or may not need to shift. If they both drop another gear the 3.55 will be in 8th running 1789 rpm ... the 3.31 equipped truck will be in 7th at 1963 rpm. Again the 3.55 is more likely to require an additional shift than is the 3.31 equipped truck but say the both drop another gear. The 3.55 will be in 7th at 2105 rpm and the 3.31 equipped truck will be in 6th at 2506 rpm. At this point you are on a fairly significant hill if all you're pulling is 16k lbs. More than likely the 3.31 is going to pull the hill the 3.55 equipped truck might need to downshift again.