Aside from any legal or logistic reasoning, I assume that you want to carry a cap-n-ball for other reasons. As you are no doubt aware, neither the .32 S&W Long or the .38 S&W are real powerhouses--though either of these cartridges in a decent gun can be quite accurate.
I have shot .31 cal cap-n-ball and they aren't very muscular even at close range. IMO, the .36 would be a much better choice; AND the pocket Navy (or pocket Police) is not all that much bigger actually. The pocket Navy and the .31 that I had were built on the same frame, with the Navy having a longer barrel (5 1/2" .36cal vs. 4" for the .31cal).
The only real advantage I can see in the .31 is the ability to shoot #0 buckshot (.320 diam.) which is much less expensive in bulk. Also the .31 uses less powder--which isn't necessarily a good thing if you are wanting some stopping power.
You don't happen to have one of the "cut-down" versions of the .44 cal guns do you? I know Cabela's used to sell them. OR... if you had (or could get) one of the .44 cal 1851 Navy pistols really cheap; you could cut that one down on your own. (or have a gunsmith or machinist friend do it). Even the (weaker) brass framed gun would be fine for this purpose since shooting a moderate load would have plenty of power for the task you have planned.
Also remember that the .38S&W is throwing approx. TWICE the weight of the .36 cal. round ball, and THREE times the weight of the .31 round ball. IIWM, I'd be thinking about your options a long while before committing to change.
Regards, and choose wisely,
WV_Hillbilly