Your biggest concern with buying a scope for a smokeless muzzle loader is... can it hold up to recoil well.
And typically the glass will be on par with the scopes ability to handle the recoil of a SML, in those types of scopes.
That should be your #1 concern...can this scope handle the recoil of a SML?
I've had a couple of Leupold VX3's let loose on me after a while.
Your best bet is to stick with the heavier tactical style scopes with a 30mm tube, and a robust body.
I have 3 Sightrons on my SMLs, and rotate a Burris XTRII, in the mix.
I also bought a S-TAC 4-20x this year , but I only have around 100 shots on it with a SML. So the jury is out on that scope still...
For the money it's hard to beat the Sightron SIIIs, but the power range can be limiting. Lots of high end magnification, but not so much on the low end.
A good bet would be the Sightron S-TAC 2.5-17x if you want a little lower power range.
I've read that is the only S-TAC that has the same glass and consturction as the SIII's do.
The S-TAC 3-16 and 4-20x, do not have the same glass and construction as the 2.5-17 do, for whatever reason. The cheaper price of the 3-16 and 4-20x, reflect that.
The Nightforce NXS, and Bushnell Tacticals have held up to the recoil of a SML as well.