Up here, the 50 cal is most common among once a year hunters simply using the muzzleloader as a chance for a couple of extra weeks hunting in the special season.
But among guys who shoot muzzleloaders year around, 54 cal is a lot more common. They certainly have a 50 or two, but they're likely in a corner or a closet gathering dust.
And over the last 5 years or so the 58 is taking over. As fast as guys can find a suitable 58, their 54's are joining the 50's in a dusty corner.