You might want to post this on the muzzleloader forum, but I believe the main difference is legal. The huntsman, since it can have shotgun barrels installed, had a different legal classification than just "muzzleloader" some some "muzzleloader only" places couldn't buy/sell them. I believe the sidekick can't have additional barrels so they're legally muzzleloaders... :roll:
Also, I believe the limit for a hunstman is 100 grains, and the sidekick has a magnum version that takes 150 grain.
I don't know for sure, I read it, but I might be dreaming. If you post on the nef muzzy board they'll fix you right up! Or, you can always call NEF direct and ask them.
later,
scruffy