Barrel length is just a matter of personal choice. I'd get the 36".
The list is taken from Connor, "Sucess in the Fur Trade", and he got them from the orders of Pierre Chouteau Jr. & Co., for the Upper Missouri trade.
1850
W. Chance Son & Co., Birmingham
160--36 inch Barrels
110--42"
10---48"
Schuckard & Gebhard [importers],
Belgium guns
60--30 inch barrels
60--33"
60--36"
40--42"
English guns
300--30"
200--33"
200--36"
1851
W. Chance son & Co.
120--30"
170--33"
230--36"
40---42"
30---48"
Edw. K. Tryon, Philadelphia
47--30"
3---36"
1852
W. Chance Son & Co.
150--30"
150--33"
250--36"
30---39"
40---42"
30---48"
1853
W. Chance Son & Co.
60--30"
50--33"
90--36"
20--39"
1854
W. Chance Son & Co.
150--30"
150--33"
180--36"
80---39"
40---42"
1855
W. Chance Son & Co.
150--30"
150--33"
120--36"
20---42"
The above is from Connor, "Success in the Fur Trade". He took it from the microfilm of the PCJ&Co. records. Notice that the 30" and 33" barrels get increasingly popular over the five years, while the 42" barrels are ordered less and less. It's very surprising that the 48" barrels were still available as late as 1852.
Now, as for resons why those barrels were shortened even further:
"When running buffaloes the hunters do not use rifle-patches but take along several balls in their mouths; the projectile thus moistened sticks to the powder when put into the gun. In the first place, on buffalo hunts, they do not carry rifles, for the reason that they think the care required in loading them takes too much time unnecessarily when shooting at close range and, furthermore, they find rifle balls too small. The hunter chases buffaloes at full gallop, discharges his gun, and reloads without slackening speed. To accomplish this he holds the weapon close within the bend of his left arm and, taking the powder horn in his right hand, draws out with his teeth the stopper, which is fastened to the horn to prevent its being lost, shakes the requisite amount of powder into his left palm, and again closes the horn. Then he grasps the gun with his right hand, holding it in a vertical position, pours the powder down the barrel, and gives the gun a sidelong thrust with the left hand, in order to shake the powder well through the priming hole into the touchpan (hunters at this place discard percussion caps as not practical).
Now he takes a bullet from his mouth and with his left hand puts it into the barrel, where, having been moistened with spittle, it adheres to the powder. He dares never to hold his weapon horizontal, that is, in position taken when firing, for fear the ball may stick fast in its course, allowing sufficient air to intervene between powder and lead to cause an explosion and splinter the barrel. So long as the ball rolls freely down there is no danger. Hunters approach the buffaloes so closely that they do not take aim but, lifting the gun lightly with both hands, point in the direction of the animal's heart and fire. They are very often wounded on the face and hands by the bursting gun barrels, which, especially when the weather is extremely cold, are shattered as easily as glass."
Rudolf Friedrich Kurz, Fort Union, 1851