I have Blackhawks in both 4-5/8" and 7-1/2" barrel lengths. The long barrels have the capability of more precise bullet placement at long range or from a rest, but for the kind of shooting you mentioned, barrel length doesn't matter. You'll probably find that you do more shooting with the ACP cylinder, and most ACP loads are optimized for a 5" or shorter barrel anyway.
I also like the longer barrels not only because they use the powerplant of the cartridge more effeciently, but for the better handling. The short guns CARRY better, but don't hang in the hand for me as well as the longer tubes do. FWIW, my Blackhawks are in all calibers from .357 to .44 Mag. My 7-1/2" .45 Convertible shoots better by a gnats' hair than my pard's 4-5/8" .45, but I've got a slightly better trigger, so it should. My pard carries his while guiding, so he likes the short gun.