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

I drive a 2012 Job 2 6.7 diesel. It engages engine braking when tow haul is turned on and no other time. In tow haul if you press the brake petal more than slightly, it will down shift to cause more engine braking. I understand that later years (maybe around 2015) the engine braking and tow haul modes were separate switches so one could engage the higher shift points of tow haul without engine braking involving aggressive down shifts. When tow haul and and cruise are both on the engine will downshift aggressively to try to maintain the cruise speed setting when gaining speed down hill. As said above, I'm in that model year range where engine braking was not very effective. However, if one sets the cruise about 10 mph less than the maximum speed that you want to achieve going down the mountain it will usually hold it back to that +10 setting. At least it will going down the South side of Mount Eagle Mountain. I don't believe that a 2012 does any automatic trailer braking. I have no idea how later years behave, but I cannot imagine that Ford would keep engine braking from working if one was driving without using cruise. Oh, and I was only indicating that I shut off cruise going down short hills where I'm only going to gain a few mph to keep the down shifting to half of what it would be otherwise. It will still down shift going up the other side.

Apologies for getting away from OP's original question. Again, either axle ratio will be fine but I would choose the 3.55 because the 10 speed top gear ratio is so low.