Mine never have problems switching between mice and asf. I do tend to feed mostly asf though as its their natural prey. If you can breed your own or find a reliable source i would feed asf, no doubt about it.
They never out grow ball pythons, a fully grown asf is plenty for a meal imo. They are certainly just as nutritious but I've heard rumors they may be more nutritious too. Nothing to ever confirm that though. I breed asf so feel free to shoot me a message any time.