I would never say that an in-line is more accurate then a traditional muzzleloader. Accuracy is a matter of what loads you work up for your rifle, and how much you practice. I own both, in-lines and traditional rifles. Both will shoot exceptional groups.

I agree you have picked a great caliber in the .54 class. Look at the Thompson Center, CVA, Tradition, A&H, and the Lyman rifles. They are the best in tradition style. I am not sure how many of them offer a .54 caliber anymore. The twist you will pick will for the most part decide what kind of projectile you will shoot, although there are some rifles that shoot about anything you stuff down them.