Re: .40 S&W or .45 ACP for song dogs???
I can tell you right now that at 50 yards the 155 gr bullet out of a 40 SW will not drop enough to matter even on a coyote. I shoot one every day that I am at work. You can hit a human sized target in the chest at 100 yards without it dropping out of the chest zone and that is out of a 3.5 in barrel.
BUT at 50 yds you are going to have to be pretty dang smooth on that trigger and focus on that front sight. Might be a little hard on a small skittish animal.
Both will work if you can shoot the pistol proficiently. It is not the best thing to be using for coyotes but will it work? Yes, if you do your job it will do the rest. I will tell you this though, if you are off 5 inches to the right or left of the chest area (which is easy to do at 50 yds). You may hit the coyote but it's gonna run off and you will never find it.