first off, muzzleloader's don't have the ability to reach out as far as a rifle and have more arc in the bullet path so to me a really high magnification scope isn't necessary.
i also prefer a scope that includes the little hash marks or "bullet drop compensator" markings when you look through the glass as it helps gain consistency when getting out past 100 yards and using holdover.
for these reasons i really like the vortex i have on my muzzleloader and would recommend it but there are definitely many other great choices out there that you may prefer as well. good luck!