When dealing with moose or other extremely large animals for that matter I firmly believe bigger is better. Especially with round ball. But one consideration is accuracy. I believe both of the calibers will and can get the job done if you place the ball properly from a reasonable distance. But I would look at accuracy and what powder charge the ball from both calibers would accept.
For instance if my .58 shot best with 70 grains of powder but my .54 shot best with 90 grains, I would sure be looking at ballistics of that ball. Both are accurate but in large game you want penetration, so which would penetrate best.
I commend you for attempting this. When I bear hunted my friends wanted to lock me up because I was hunting them one day with a .54 caliber with round ball and 90 grains of powder. I had not purchased my .58 at that time. And I carried a .44 mag revolver as security. I later changed to a REAL conical just to shut them up kind of. But I really had no doubt that the round ball would have got the job done. But never seeing a shooting bear, I never got a chance to prove that.