Interesting question.
I, too, thought of the 38 as a better choice, but he gave a top-year of '76, and I don't know if Colt was chambering theirs in that, yet. Supposedly, the 38 'hits harder". 180 grain 40 caliber slug at 1100 does seem like it might hit harder than a 200 grain 43 caliber slug at 900.
But, of those two choices, I'd go with 44. While the 45 is, I believe, a better pistol round, it's not THAT MUCH better, and it seems more likely that, in some out-of-the-way trading post, if they only had ONE caliber of ammunition, it would be the most common rifle ammo. So having a pistol chambered in that rifle cartridge would be a plus.