I have used a 3-12burris black diamond, 4-16 nikon, and several 3x9's. The 3x9 is plenty for a muzzle loader.
I had a 3x9 zeiss conquest on my sidekick this year and was very pleased with it. The conquest has loads of eye relief. IMHO spend the money on the optics do not short change yourself. There is lots of good scopes out there. A nikon buckmaster would be good, a 3200 series B&L,..
Sounds like you have a nice muzzle loader coming