.54 balls and bullets cost more.
.54 rifles recoil more
Placement being everything, and Practice=Placement, you are going to get get more practice with a .50.
A conical ideally needs a faster twist around 1:32 or faster.
Depends upon the length of the conical, don't it?
Very long projectiles need to be spun faster to keep them flying point forward.
Shorter projectiles, not as much.
Round balls, hardly at all.
My 1:48" .50 capgun does just fine (4" at 100 off sticks) with both Great Plains 385gr conicals or 370gr Maxiballs ...... I can't hold to that with my 1:66" .45 rocksmasher and .440 patched roundball .... glacial locktime and that damned flash in front of my face......
eta: the .45 is far more pleasant for target shooting, as well as bing challenging to operate successfully. It does not make for a good blood trail on a deer though.....