I guess its what works for the individual. I have shot and hunted with every caliber from 40 to 72 and for hunting big game especially bare and deer I use my .54 with 129 grains of RS and a .535 ball with a ticking patch lubed with coon grease. Works best for me.
I find the higher velocity to give both the best accurate long range and the best hitting power. I know there will people who quote number to disagree and people who think a bigger ball shot slower works better but after trying them all on deer and the 58 and 54 on bear this is what works best for me.