Loose powder is more consistent pellets have more weight irregularity.
"Which powder is cleaner?" There is no question that Blackhorn is the cleanest but only Black Mag and 777 pellets are more expensive so some don't like the price.
There are a couple other things that enter in to the question in my experience, Pyrodex has a soft residue which you can load a couple shot over and still maintain accuracy; 777 needs to be swabbed out real good to maintain accuracy and if you are among the group that gets crud ring that can be a problem.
With that particular gun some have found it best if they use CCI209M or Federal 209's for Blackhorn. Lee