Whichever one feels best to you.
Tapered octagon barrels can point and balance very well. Round barrels are usually cheaper.
I wouldn't get a 24" in .45 Colt, but that's just me. I see the .45 as a better short-range popper than an ideal long-range rifle.
I have a 24" heavy round barrel .22LR lever gun that I really like, and a similar 20" tapered octagon .22 that I also really like.